Support for Proof-of-Stake chains

[sondreb]

I have made modifications to bitcoinjs-lib so it can support Proof-of-Stake chains, and since it is not wanted to have support for different networks built directly into that lib, I'm going to make a PR for this library which means I will extend the object structure with a new field.

Suggested name: isProofOfStake (boolean).

Too explicit or anyone have better suggestions?

[sondreb]

The x42 Proof-of-Stake chain was added with the property "isProofOfStake", so I'm closing this as the suggested name has been approved into the repo.

A new PR for City Coin support has also been added: #103