Support Bitcoin secp256k1 signature verification

[ethanfrey]

Part of #751
Requires #780
Follow same steps as #752

Add a method that verifies a public key and signature in the format used by ethereum (compressed pubkey, RLP encoded signature). This can either be a separate verify_secp256k1_btc() method or provide a function to convert this into the same format used by cosmos and then verify it.

Here is a starting place to look at how Bitcoin signatures are DER encoded: https://bitcoin.stackexchange.com/questions/12554/why-the-signature-is-always-65-13232-bytes-long

[webmaster128]

Closing in favour of #798