Memoize decorator doesn't work with tmux segments

The `[memoize](https://github.com/Lokaltog/powerline/blob/develop/powerline/lib/memoize.py)` decorator that's used to cache the results of segments doesn't work with tmux, since the caching mechanism stores results in memory and is erased after the script has been evaluated.

My solution adds a "filename" argument to the memoize decorator, which should contain an absolute path to a file that will hold the pickled cache. Might want to change this to always put the file in the tmpdir.