Refactor and merge apps/topology to trema.
* Merge discovery process etc.
* Moved show_{topology,switch_status} to examples/topology.
* Minimized show_topology example.
* Add sample .conf files for examples.
* Add external attribute to show_switch output.
* Rename libtopology -> topology.  C API. (#3)
* Add event filter operation. (#2)
* Auto start topology daemon. (#5)
Add Topology minimal Ruby API