Cross platform cachedir, again.

Another attempt at getting a cross-platform and (on Linux) XDG compliant cache dir.

Squashed commit of the following:

- Correct usage of os.getenv (thx @tkf)
- Use `~/.cache/jedi` as default fallback, not `~/.jedi`
- Updated cache location for OS X
- Added expanduser to cachedir
- Cross platform cachedir, again.