Message signing and altcoins

Chained on #180, but not dependent.

This pull request centers mostly around adding support for alternative [`strMessageMagic`](https://github.com/dogecoin/dogecoin/blob/6e5881a000e5b926b93143c70cd2fb00239d78f8/src/main.cpp#L72) constants which often vary between alt-coin implementations.

The implemented list of constants can be [seen here](https://github.com/dcousens/bitcoinjs-lib/commit/4d182db72262ff213ec47fd98839b459ac944c96#diff-9e60abeb4e2333a5d2f02de53b4edfacL2).

It should be noted that the length has been included at the front of these constants as inline hex constants `\x18`, this is to save on adding the generic variable integer code that would be needed to do it.

Also, for all signature data, I've opted towards base64, as that is easily verifiable against `bitcoin-qt`.

Comparison link: https://github.com/dcousens/bitcoinjs-lib/compare/eckey1...mesnet