The Android NDK was recently removed from gomobile, leading to our Android
builds failing. Starting from https://go-review.googlesource.com/#/c/35173/ ,
gomobile requires a locally installed NDK. This PR ensures that travis installs
that too before running the build steps.

core: removal of dead-code

Removal of dead code that appeard as if we had a consensus issue. This
however is not the case as the proper error catching happens in the vm
package instead.

cmd,eth,les,internal: remove natspec support

* core: Made logging of reorgs more structured, also always log if reorg is > 63 blocks long

* core/blockchain: go fmt

* core/blockchain: Minor fixes to the reorg reporting

accounts, mobile: make account manager API a bit more uniform

common/hexutil: fix EncodeBig, Big.MarshalJSON

The code was too clever and failed to include zeros on a big.Word
boundary.

travis, appveyor, build: add source spell checking

- Use defined constants instead of hard-coding their integer value.
- Allocate secp256k1 structs on the C stack instead of converting []byte
- Remove dead code

tests: unskip test

core: remove support for Olympic network

core: improve import log alignment

vendor: update dependencies

cmd/utils: fix comma-separated --bootnodes

The Azure SDK doesn't support Go 1.5 anymore. We can't upgrade it until
Go 1.8 comes out.

all: fix ineffectual assignments

params, VERSION: start 1.5.7 release cycle

params: stable version 1.5.6