Implement WorkingCopy::getUntrackedFiles.

Currently, the WorkingCopy::getUntrackedFiles method returns only a
hardcoded empty array. This commit adds an implementation for this
method that parses `git status` output and returns the list of
untracked files as array.
Merge pull request #72 from martin-helmich/feature-implement-getuntrackedfiles

Implement WorkingCopy::getUntrackedFiles.