RSA signature support

[nrktkt]

should be easy to support all the variants since they only vary in hash size but use the same keys

after completing, open a PR to https://github.com/jsonwebtoken/jsonwebtoken.github.io/blob/master/views/website/libraries/15-Scala.json to indicate RSA support has been added

[tclostio]

Giving this one a whack.

[nrktkt]

This is now implemented for verification, but still needs signing to be implemented in https://github.com/blackdoor/jose/blob/master/jose/src/black/door/jose/jwa/RSAlg.scala.
Which makes it a very well contained feature to add.