Start OAuth2 compatibility

 - Reworked RedditClient.login() method to take a Credentials object
 - New class OAuth2RedditClient that extends RedditClient. Can login, logout,
   and get information about the authenticated user. So far the only Reddit
   apps that are supported are 'script' type apps.
 - credentials.txt has been replaced by credentials.json
 - New method in HttpClient: executeWithBasicAuth() that takes a username and
   a password
 - Renamed HOST_HTTPS_SPECIAL to HOST_SPECIAL
 - New test class: 'OAuth2Test'
Merge branch 'master' of https://github.com/thatJavaNerd/JRAW