lib secp256k1.cr v0.1.3

this is a feature release that allows for generation of bitcoin and ethereum addresses.

changes:

• add cryptographic hashing functions #9
• add bitcoin and ethereum address modules #9
• allow for export of unprefixed uncompressed keys for ethereum #9
• add tests for hashes on bytes #9
• implement eip-55 test cases #9
• bind mpz_powm_sec from libgmp #11