Improve key store crypto

replaced path with platform aware filepath module

* Also fix address types post-rebase

* Remove key header from unencrypted key file format and replace
  it with a version field
* Change encoding of bytes in key files from base64 to hex

This reverts commit a94d4ba0b53c4558ab838aaed635a2ff66ddfa53.

* Add key header containing key version, kdf and kdf params
* Store key header as JSON in the key file
* Read in KDF params from key header
* Include key header in MAC calculation and MAC verification

* Change MAC-then-Encrypt to Encrypt-then-MAC
* Change AES256 to AES128
* Use first 16 bytes of KDF derived key for AES and
  remaining 16 for MAC

xeth, rpc: implement eth_estimateGas. Closes #930

Closes #880

compiler, cli/js: fix solidity compiler tests

eth, ethdb: lower the amount of open files & improve err messages for db

Closes #880

miner, cmd/geth: miner will not ignored owned account transactions

Miner does not ignore low gas txs from accounts that are owned.

cmd/geth, miner, backend, xeth: Fixed miner threads to be settable

Miner threads are now settable through the admin interface (closes #897)
and specify 0 CPU worker threads when eth_getWork is called (closes #916)