Build cleanup

Update gem dependencies + code changes to support. Biggest changes are:
* rspec2 to rspec 3 + fix all deprecation warnings
* rcov to simplecov
* JWT update to 1.0, required some small changes to handle change in return values

Also
* Simplified how the gemspec/gemfile files are maintained to remove duplication and extra checkins. signet.gempsec is now the source of truth.
* Dropped 1.8 & 1.9.2 from travis config
* One minor bugfix creeped in -- added support for form-encoded responses during code exchange (#27) for Facebook & other providers that don't return JSON.