[WIP] Mutant

Opening early to allow discussions.

### Changes

* Adds mutant rake task
* Sets up mutant in "run all tests per mutation" (no test selection, fast enough for this small project)
* Set expected coverage threshold in rake task. It'll fail above or below that threshold! (Needs manual adjustment if coverage increases!)

### TODO:

* [ ] The above adjustment will be pushed down to a mutant config option before PR should be merged.
* [ ] Bring back simplecov integration in `spec_helper.rb` and guard it in a way it does not interfere with mutant. 

### Questions:

* Hook mutant rake task on CI and make it fail if coverage is not meet? I like it that way, but some people (greez @solnic) prefer not to fail on violated metric tools.

The current mutant report looks like this:  https://gist.github.com/mbj/10641431

This PR is not intended to be merged right now. I just open it early to give interested participants a context for discussion.

My intention is to identify more TODOs, and fix them before asking for the real merge.

@avdi If you think that WIP PR is too much noise, I'll reopen it against my fork. Feel free to close it in this case.