[WIP] Implement basic tagging system for testsystems

[jchodera]

Addresses #190

This PR begins to implement a system where we can add keyword tags to each system to enable autodiscovery of testsystems that have useful properties, like are quick to run on travis (or too slow to run on travis).

Instead of having the initializer define the tags, it would be useful to be able to query this without having to actually construct the object, since that construction itself can be slow. I'm still trying to think of how best to tackle that.

[jchodera]

Any thoughts on this approach from @Lnaden @andrrizzi @pgrinaway?

[andrrizzi]

Nothing particularly smart. The only thing that comes to mind is to maintain a dictionary tuple_of_tags: test_system, but you'd still have to implement the infrastructure to cycle over all test systems if you only need to match a subset of tags that compose the key.