Fix cabal file, reorder the 'extra-source-files' section
Export the IntDate and StringOrURI types

* Extrace the base64 encoding/decoding functions into a non visible module
* Removed the functions/types only exported in test mode
* Export the IntDate type, add a StringOrURI type and accompanying functions
  that will create those types
* Documented the JWTHeader fields
* Change the JWTClaimsSet fields from (Maybe Text) to (Maybe StringOrURI)
Rev version to 0.2.0