* Before, when code inside the isolation failed outside the tests we
counted this as a mutation kill.
* Now we report them explicitly as not killed mutations. As they where
not killed by the tests.
* 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]
