This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.
* break out cache into cache.py
* break out cache-tests into test_cache.py
* fix broken cache tests
* replace non existing assert calls with self.assertEqual
* fix path for page caching test (was invalid)
* cleanup test code
* restructure generate_context in Article and Path Generator
* destinguish between valid/invalid files correctly and cache accordingly
* use cPickle if available for increased performance
