Support Decimal serialization

AuthProxy would return Decimal objects, which is great.
But could not JSON-encode Decimal objects, so things
like this didn't work:

    balance = connection.getbalance()
    connection.sendtoaddress(foo, balance/2)

The fix is a little EncodeDecimal helper function.

Tested in some Bitcoin smartfee regression tests I'm writing.

Built on top of #29.