Update rapidsms-tropo for RapidSMS 0.14.0

    * Interface updates to work with RapidSMS 0.14.0
    * Bulk sending is supported.
    * Not compatible with RapidSMS<0.14.0 or Django<1.4
    * Add tests!
    * Add tox.ini
    * Drop dependency on tropo library, which hardly added
      anything
    * Use https: when calling Tropo
    * Check validity of incoming requests from Tropo better.
    * Bump version to rapidsms-tropo 0.2.0
Don't need get_backend() utility
Bump release date for 0.2.0 to 2013-05-20
Merge branch 'release/rapidsms-tropo-0.2.0' into develop