significant progress towards bitcore compatibility

Cryptography updated to use bitcore methods rather than bitcoinjs.
Get test vector 1 working in node
make things work in the browser by fixing sha512

...had to use jsSHA package to do SHA512 in the browser. Unfortunately it is
quite slow compared to node.
all vector 2 tests work
remove obsolete commented-out code
add convenience constructor for making new bip32s

Added the ability to create a new master bip32 with new private key and chain code. The way this works is like this:

var bip32 = new BIP32('mainnet');

or:

var bip32 = new BIP32('testnet');
move version constants to networks.js

...with all the other network-specific constants.