Added static file helper

Example usage test-case:

    class StaticFinderTestCase(LoaderTestCaseMixin, unittest.TestCase):

        paths = (
            os.path.join(os.path.dirname(__file__), 'test-packages', 'media-package'),
        )
        
        def setUp(self):
            self.loader = loaders.ModuleLoader()
            super(StaticFinderTestCase, self).setUp()
        
        def test_load(self):
            modules = self.loader.load('testplugin')
            self.assertIsNotNone(modules.findFile('templates/bar.html'))
            self.assertIsNone(modules.findFile('templates/err.html'))