Support OP_CHECKSIG

[JornC]

OP_CHECKSIG is currently mocked to always return true.

To make this work we need a pure-java (non-native) or JS library implementation of the secp256k1 ECDSA curve. This is ugly, because it is highly probable such a library is not as well tested as OpenSSL or libsecp256k1 - but it is highly desirable to be able to do (and show!) signature verification on the client, and sufficiently safe because we are not signing anything (or otherwise using it for any monetary purpose).

To be a good boy in class, though, it would be desirable to have the Bitcoin Core node do the verification aswell (no support for this through JSON-RPC - as far as I can tell)