add_flow and del_flow commands added

[nomorecaddy]

added these commands to the webservice demo which adds / deletes a flow on the switch

[MurphyMc]

Hmm... I had specifically left out individual add and remove flow commands with the idea that managing state incrementally is difficult. Instead, I just had the set_table command. This forces the client to be aware of the complete state they actually want, but it also (theoretically, anyway) gets rid of ugly race conditions and ambiguity.

[MurphyMc]

Just a note that if this is going to proceed, it should target dart.

[MurphyMc]

I've decided I'd merge this with a few changes:

1. As mentioned, list_switch() should just be an optional parameter to list_switches(), not a whole new method.
2. It should target dart.
3. POX doesn't (or at least shouldn't) use the term "flow" to mean "table entry", so the commands should be named something like add_table_entry() or add_entry(). And actually, OpenFlow 1.0's "add" is ambiguous, since it can also be a "modify". So maybe set_table_entry()?