ECPair: replaces ECKey/ECPubKey
ECPair: find network from constants
ECPair: throw if non-boolean compressed flag given

Also adds test for network option
ECPair: use ECPair.curve internally
ECPair: move rng to optional params object
ECPair: throw for invalid network
tests: remove unused fixtures
ECPair: rename *PublicKey to *PublicKeyBuffer
Merge pull request #290 from bitcoinjs/ecpair

ECPair: replaces ECKey/ECPubKey