add CheckWIF
split TestExport into TestExportWIF and TestExportAddress in unit tests
add derive helper method to PrivateKey, add derive unit test
add ToBytes() and FromBytes() to PublicKey and PrivateKey
add static init of secp256k1 curve in btckey package
rename ecdsa to elliptic since the module only does elliptic curve arithmetic
add PrivateKey FromWIF; consolidate key-related test vectors in unit tests
fix PublicKey ToBytes()/FromBytes() to include/expect prefix byte
add zero padding when necessary to PublicKey and PrivateKey ToBytes()