py3k fb303 compatibility

Even though thrift doesn't support py3k, let's use six's iteritems
instead of iteritems directly.
ExportedValues API

This is a popular API like counters, but for arbitrary data.

The current implementation is only on VService, but I could see this
being useful to be implemented more like Counters.

That said, I'd like to clean up the counters/options/etc implementation
before introducting other things that might look more like them.

Test Plan: Added some new unittests to test_vservice to make it work
properly.