refactored the caching mechanism

I found the [AppDirs](https://pypi.python.org/pypi/appdirs/) package from the ActiveState people and think this solves the problem of determining platform specific cache dirs nicely (or at least in a documented way we can point to).

Access to the cache is now available to all modules via ``lingpy.cache`` and the ``QLCParser`` class was refactored accordingly.

While this change comes at the expense of two additional dependencies, I think it is still worthwhile, because ``AppDirs`` is small and easy to install and ``pathlib`` is actually part of the standard library as of python 3.4.