Add installable wsgi file

This simplifies the graphite.wsgi configuration example.

When running graphite in a virtualenv with gunicorn, this also allows running
`gunicorn graphite.wsgi` directly, without any additional configuration.
Merge remote-tracking branch 'upstream/master' into feature/wsgi
Add back the comment about preloading search index
Merge pull request #513 from brutasse/feature/wsgi

Add installable wsgi file