Raw transactions RPC + batch JSON-RPC

Here's the pull request for the RPC branch (c.f. issue #20).
It's not supposed to be merged right now, but in the second release of Peerunity.

It contains the RPC functions to manage raw transactions:
- listunspent
- getrawtransaction
- decoderawtransaction
- createrawtransaction
- signrawtransaction
- sendrawtransaction

I tested these with 2/2 and 2/3 multisig addresses.

I also added the support for batch JSON-RPC, allowing to send several requests at once.
I tested it with curl.

There are files in the "testing" directory explaining the tests I have done.