Allow mocking the local hregion as an htable.

The issue is it's not possible to wrap the existing hregion mock in a
`HRegionAsHTable` adapter since there's no easy way to make it satisfy
the needed java interface.

So mock around it instead, by fixing a single `ExecutionContext`
instance and using real `Relation` objects wrapping doubles.

Note we stub the execution context out on the `Runner` by hijacking
the thread local: the runner itself can't be stubbed since it is
frozen on construction.
Merge pull request #92 from seomoz/phs/mockable-local-relations

Allow mocking the local hregion as an htable.
`Relation#to_key_values` transforms cells back into `KeyValue`s.

I don't expect this to happen much in production, but it is handy for
tests (and pulling this out to fixture support ends up dragging out
lots of guts.)