Fine grained rspec integration

* Flattens the rspec example groups on filtering to remove redundant
  work being done on executing subtrees of already executed example
  groups
* Uses test batching to kill one mutation per isolation that results in
  a significant speedup.
* Drop rspec 2 support.

[closes #256]
Merge pull request #277 from mbj/rspec-fine-grained

Fine grained rspec integration