PEP 519 Support

[levic]

Addresses #24

• Now handles python 3.6 DirEntry and pathlib.Path
• AbstractPath probably doesn't need __fspath__() since it already extends str and anything handling os.PathLike probably has strings as a special case already (isinstance('', os.PathLike) is False), but I put it in there anyway
• Updated documentation (not that there's much to update) but didn't regenerate the html; no version bump -- I assume this would be part of a release/build step?

There is no special-case handling of pathlib or DirEntry, this is purely a PEP519 implementation (ie for python <3.6 things will continue to fail)

[levic]

(It should be noted that the bulk of the work is actually done in the python std lib: Unipath is just a wrapper to os.path. As long as you let PathLike instances through the type check then os.path takes care of calling __fspath__ on the relevant objects for you)