Add basic support for detecting coverage regressions

[glv]

We just encountered a case where a change to the app's code caused a lot of the app to be closed off to Tarantula; given the privileges with which it was logging in for the crawl, it suddenly could only see about 20% of the app. I happened to notice, but it would've been easy to miss. Tarantula needs to help detect that case.

The "right way" to do this is by integrating rcov and reporting the percentage of code covered by the crawl (and optionally failing the build when coverage is too low). But that would also slow down the crawl a lot, so a simpler alternative should also be provided. We should be able to tell Tarantula "expect to crawl at least n pages" and have Tarantula fail the build if fewer pages are found (and also prompt users to raise the threshold when the actual number is too far above that number).

[karnowski]

What about doing a simple file-system scan to list all view files under the app/views directory, and then we could report on how many of these were rendered during the run? We'd have to alias_method_chain render (which is non-trivial), but unit-controller is a good example of that.

[glv]

Nice idea. I still think it would be good to have a slow-but-thorough rcov option and a fast-but-heuristic option, but this is a great idea for the latter.