ItBit initial commit

Initial commit for itbit.com support. Fully functioning for trading, some API calls are not fully implemented yet such as deposit and withdrawals. Tests and examples are also missing yet, I have an example that tests all calls, I'll split that up and will upload it over the next week.

One trend I see with new exchanges is more complex account/wallet handling than what is currently supported by or practical with XChange, maybe something to think about for the future. For example itBit associates every authenticated action such as placing orders with a specific wallet id and you can create a large number of wallets all with their own set of balances for each currency. I currently solve this by specifying one walletId for trading via the exchange specific parameters, but it's not very flexible. Either way, maybe we can think of a way of making the Wallet/Account more flexible.
@timmolter

Actually, please leave this PR open, I forgot about the *Raw services (which is important for itBit because that provides quite some extra functionality), which I'll push next.