all: Add GPU mining, disabled by default

core/state, core, miner: handle missing root error from state.New

Update libsecp256k1, Go wrapper and tests

core, eth, event, miner, xeth: fix event post / subscription race

common, crypto: add ICAP functions

core/vm: copy stack element to prevent overwrites

cmd, core, eth: added official testnet

cmd: properly initialize Olympic for all subcommands

core, core/vm, cmd/evm: remove redundant balance check

crypto: correct sig validation, add missing unit tests of exported functions

console/history respect datadir

core: fix #1848, block receipts db entry for the genesis too

cmd/geth: dev version number

core: fixed head write on block insertion

Due to a rebase this probably got overlooked / ignored. This fixes the
issue of a block insertion never writing the last block.

core, core/vm: refactor

* Renamed ChainManager to BlockChain
* Checkpointing is no longer required and never really properly worked
when the state was corrupted.

* Moved `vm.Transfer` to `core` package and changed execution to call
`env.Transfer` instead of `core.Transfer` directly.
* core/vm: byte code VM moved to jump table instead of switch
* Moved `vm.Transfer` to `core` package and changed execution to call
  `env.Transfer` instead of `core.Transfer` directly.
* Byte code VM now shares the same code as the JITVM
* Renamed Context to Contract
* Changed initialiser of state transition & unexported methods
* Removed the Execution object and refactor `Call`, `CallCode` &
  `Create` in to their own functions instead of being methods.
* Removed the hard dep on the state for the VM. The VM now
  depends on a Database interface returned by the environment. In the
  process the core now depends less on the statedb by usage of the env
* Moved `Log` from package `core/state` to package `core/vm`.

Moved the filtering system from `event` to `eth/filters` package and
removed the `core.Filter` object. The `filters.Filter` object now
requires a `common.Database` rather than a `eth.Backend` and invokes the
`core.GetBlockByX` directly rather than thru a "manager".

eth/downloader: match capabilities when querying idle peers

core: deadlock in chainmanager after posting RemovedTransactionEvent

This PR solves an issue with the chain manager posting a
`RemovedTransactionEvent`, the tx pool will try to
acquire the chainmanager lock which has previously been locked prior to
posting `RemovedTransactionEvent`. This results in a deadlock in the
core.

core, trie: new trie

common: fix #1818, secondary datadir paths to fall back to

p2p/discover: fix race involving the seed node iterator

The strict matching can get in the way of protocol upgrades.

nodeDB.querySeeds was not safe for concurrent use but could be called
concurrenty on multiple goroutines in the following case:

- the table was empty
- a timed refresh started
- a lookup was started and initiated refresh

These conditions are unlikely to coincide during normal use, but are
much more likely to occur all at once when the user's machine just woke
from sleep. The root cause of the issue is that querySeeds reused the
same leveldb iterator until it was exhausted.

This commit moves the refresh scheduling logic into its own goroutine
(so only one refresh is ever active) and changes querySeeds to not use
a persistent iterator. The seed node selection is now more random and
ignores nodes that have not been contacted in the last 5 days.