Cross platform cachedir, again.

Another attempt at getting a cross-platform and (on Linux) XDG compliant cache dir, as discussed in #102.

```python
if platform.system().lower() == 'windows':
    _cache_directory = os.path.join(os.getenv('APPDATA') or '~', 'Jedi', 'Jedi')
elif platform.system().lower() == 'darwin':
    _cache_directory = os.path.join('~', '.jedi')
else:
    _cache_directory = os.path.join(os.getenv('XDG_CACHE_HOME', '~'), 'jedi')
cache_directory = _cache_directory
```