keybase ssh login: PEM based keys

Add bouncycastle
Only require the private key to login key based.
Code clean up

Add bouncy castle

Remove DER style keybased login

This involved removing the public key reading and algorithm selection, which is not needed anymore..

Clean up public key related code
Merge pull request #1 from chrber/feature/2.10/pemKeyBasedLogin

keybase ssh login: PEM based keys