sign/verify message bitcoind style

original code from https://github.com/scintill/php-bitcoin-signature-routines by @scintill (he just had the code to verify, I added signing).

test data is generated using bitcoind.
test data can also be verified against bitcoind by setting SignVerifyMessageTest::$againstRPC to true (and you have to change the RPC password I guess ...)
aye, just copy pasting, but I'll try avoiding it from the future (and fix these)