Daring Django

• Overall code and test refactoring, thanks to @gsakkis
• Allow the del statement for resetting cached properties with ttl instead of del obj._cache[attr], thanks to @gsakkis.
• Uncovered a bug in PyPy, https://bitbucket.org/pypy/pypy/issue/2033/attributeerror-object-attribute-is-read, thanks to @gsakkis
• Fixed threaded_cached_property_with_ttl to actually be thread-safe, thanks to @gsakkis