replacing hash() with md5 from hash lib

this fixes eve-val/evelink#168, tested with python 2 and python 3.

hashlib md5 is a bit picky what it takes.. I tried bytes and strings but looks like this is the easiest thing that works with py2 and py3
This docstring is intended to target anyone who might wish to use the cache, not just EVELink internal code - so it should describe the actual value being put here in generic terms that anyone could use. (For instance, "a string hash key".)