ECKey WIF only

Depends on #179.

The test `Message.signing.'testnet address'` was removed, as the signing process is agnostic of a network parameter for now (this will change when cross-network magic message constants are added `network.js`).

The only controversial functionality change in this pull request is the removal of `ECKey.*Buffer/*Hex` functions.
This is change is suggested as these functions gave the impression of serialization of an `ECKey`, when really they only serialized `ECKey.D`.

These formats have therefore been removed in favour of always using a sane serialization format (WIF).

If the user needs the previous functionality, simply use `privKey.D.*` instead of `privKey.*`, as BigInteger supports the `*Buffer/*Hex` functions as expected.  So this does not pose any real inconvenience to the user.

More extensive test fixtures for `ECKey` were [also added](https://github.com/dcousens/bitcoinjs-lib/blob/b63a628b8cdfd4c0843b73b3626169e312ad83a2/test/fixtures/eckey.js), testing exceptions where expected and that the calculated public keys are as expected.

Comparison link: https://github.com/dcousens/bitcoinjs-lib/compare/txscript...eckey1