Add support for the OAuth2 application-only flow supported by /1.1/search/tweets, /oauth2/token and others.
Merge pull request #160 from nicksloan/master

Fixes sixohsix/twitter#154: application-only authentication with oauth2 support
Fixes sixohsix/twitter#154: application-only authentication with oauth2 support. Tested on python 2.7 and python 3.3.