Add KeyPair from mnemonic

[nullcount]

I understand that each one-time use KeyPair should be stored by the clients -- not the library itself. However, the library could assist in key-recovery efforts by supporting the generation of child key pairs from a parent seed phrase mnemonic. That way, if clients lost access to their secret keys, it is still possible to recover those keys using the mnemonic.

Usage might look like this:

keypair = KeyPair.from_mnemonic("bacon bacon bacon bacon bacon bacon bacon bacon bacon bacon bacon bacon")