Update of pyjwt to support RSA keys

Hi, 

For a project I am working on I have update you pyjwt library to support the RSASSA-PKCS1-v1_5 signature algorithm using SHA hashes. You might be interested in merging this upstream if you find the changes to be correct and interesting, and possibly update the version on Pypi? 

Since we when using symmetric public/private keys needs to verify the signature rather than just generating the same signature again and compare, I have splitted the signing_methods into two different maps, one for signing and one for verifying.

Kind regards,
Åsmund