Removed biggest memory sieve - Finder (mostly fixes #385)

By some weird reason, Finder eats memory like crazy. Removed it in favour of iterators. This
also eliminated big dependency from the core.
Sort feature paths before testing them
Simplified FilesystemFeatureLocator::findFeatureFiles
Anchored the end of the regex matching the feature paths
Merge pull request #387 from Behat/performace/finder-removal

Removed biggest memory sieve - Finder