Removed useless suggest information from composer.json
Initial commit for 0.2 branch

Changes are mostly internal, a huge unification and factorisation work
is being done in order to make implentation using various backends
easier API remains almost the same.

Most notable changes:

 * Renamed the PubSubInterface to BackendInterface

 * Getting rid of redundancy in message handling into various
   implementations and unify everything into the BackendInterface

 * In the long term, remove every specific implementations except
   the BackendInterface in order to simplify implementation

 * Added preliminary method signatures for sending a single message
   to multiple channels

 * Drupal 7 / DBTNG implementation seems OK so far, memory
   implementation has not been fixed yet