add .gitignore
mv assets/ to site/
ship scratch.png with weave-minimal (privacy etc.)
export application object when using gunicorn or wsgi, fix #20

For higher concurrency (if possible at all with SQLite), gevent will be used if
installed (`pip install gevent`). Furthermore, `weave-minimal` exports an
*application* object for uWSGI and Gunicorn, e.g.:

```bash
$ env ENABLE_REGISTRATION=1 DATA_DIR=/var/lib/... gunicorn weave -b localhost:1234
```

Do *not* use multiple processes to run `weave-minimal`. The code does not
acquire inter-process locks on the database and I have no plans to add a IPC
concurrency pattern to the rather simple code base (programmer's lame excuse,
I know).