Add support for Rdio OAuth 1.0a and 2.0

Enable with:

social_auth.backends.contrib.rdio.RdioOAuth1Backend
social_auth.backends.contrib.rdio.RdioOAuth2Backend
Document Rdio backend
Merge pull request #614 from dasevilla/rdio-oauth

Rdio OAuth 1.0a and 2.0 support