Cached-property

* Added conftest.py to manifest so tests work properly off the tarball, thanks to dotlambda
* Ensured new asyncio tests didn't break Python 2.7 builds on Debian, thanks to pydanny
* Code formatting via black, thanks to pydanny and ambv

* Added asyncio support, thanks to vbraun
* Remove Python 2.6 support, whose end of life was 5 years ago, thanks to pydanny

* Validate for Python 3.6

* Drop some non-ASCII characters from HISTORY.rst, thanks to AdamWill
* Added official support for Python 3.5, thanks to pydanny and audreyr
* Removed confusingly placed lock from example, thanks to ionelmc
* Corrected invalidation cache documentation, thanks to proofit404
* Updated to latest Travis-CI environment, thanks to audreyr

* 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

* Regression: As the cache was not always clearing, we've broken out the time to expire feature to its own set of specific tools, thanks to pydanny
* Fixed typo in README, thanks to zoidbergwill