Add pubkey recovery API

[webmaster128]

• Implementation
• Return uncompressed pubkey instead of compressed
• Use Ethereum address to identify signer in example

Closes #765

[maurolacy]

Great work.

Main issues I have:

• Passing of the recovery parameter in the recover pubkey API. Wouldn't be better to handle all that internally?
• Error types proliferation (and associated boilerplate).
• Asking for and validating the signer's address. Perhaps that could be another API / contract call?.

[maurolacy]

Nice helpers.