Refactor riak_context and Breakout Event Library

- riak_context and riak_event have been refactored into:
    - riak_config = Configuration set at startup and does not change
    - riak_connect = One per socket
    - riak_operation = GET/PUT/List Keys, etc Riak Operation for message-specific state
- Added `adapters` directory for event library-specific support.  Currently only libevent.  Might add another one in my free time.
- Pushed libevent-specific stuff into example as well
- Moved async callbacks into examples since they are specific
- Added boring simple socket support for synchronous version
- Switched `_RIAK_DEBUG` back on for testing
- Addresses https://github.com/basho/riak-c-client/pull/23, https://github.com/basho/riak-c-client/issues/16 and https://github.com/basho/riak-c-client/issues/13