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

go get github.com/gordonklaus/ineffassign
ineffassign .

We don't use this anymore.

les: fixed selectPeer deadlock, improved request distribution

core: fix race condition in WriteMipmapBloom

console: fix error message in faux JSON-RPC responses