Added some convenience features

I added two features for convenience:
1) datetime conversion for 'iat' and 'nbf' as well.
Previously 'exp' was the only thing supported.
These two claims are the two standard claims also in time format (http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html#issDef).

2) key value for RS* algorithms are converted automatically from a PEM string.
I don't want to fuss around with the details like format conversions, importing packages, etc.
Since Crypto packages is already imported and handled internally, it appears a good idea to have them handle the key conversion as well.
Prevent build failure in Python 3.x version
Added test cases for the change
Merge pull request #29 from gullpong/change

Added some convenience features