Increased isolation of Message.* Tests

This pull request further isolates the Message.* tests as done in d7eb87433c. 

The main point of this pull request is to show how `compression` is merely a flag of the output format of `ECPubKey.toBytes`, and has no real impact except on how the public key hashes to an address.

This is easily shown (and tested) by lines 14, 15 and the test case on line 50. 

Like the rest of the API in Bitcoinjs, byte arrays have been preferred to avoid constant data format interchange; especially since, although the hex format for signatures is convenient, it was still different from the default format in bitcoind (base64 signature output).