Major changes made to the JPImgurClient class

Now there is two clients, one to manage tokens, the other one to
communicate with the API.
The singleton has been dropped since the base URL is configurable, it
would be stupid to return an older instance with a different base URL
specified.
Merged unit tests files, all the test targets use the same files
Renamed constants according to the coding guidelines written by Apple
Implemented authenticateUsingOAuthWithPIN:success:failure: method
Added "Client ID" and "Client Secret" keys to the plist used for tests
Fixed some OAuth URLs
Added some unit tests