more reusable nosetest engine

Summary:
The NoseTestEngine class has some usefull code for parsing nosetest, but it assumes a very specific code/test
layout. I've pulled this logic abit apart, which lets me reuse the nose-related parts with my existing
project layout, by having my custom engine just call runTests() with the relevant paths.

Test Plan: Run on a customized project with coverage, see coverage results.

Reviewers: epriestley, roman.barzyczak

Reviewed By: epriestley

CC: seporaitis, aran, Korvin, zeeg

Differential Revision: https://secure.phabricator.com/D5921
Merge branch 'master' of http://github.com/facebook/arcanist