Complete overhaul of the token handler

- Fully documented all methods.
- Simplified complex codepaths in the JsonWebToken and removed duplicate and unnecessary code.
- Fixed bug where duplicate claims were being added.
- Moved to using the ClaimTypes constants instead of manually defined constants.
- Explicitly map returned data to common claims.
- Renamed properties in the ValidationHandler to map with Auth0 properties.
- Bumped NuSpec version to 1.0 and added myself as a contributor.
- Modified readme files to reflect refactoring.