Add GridFS support

[dcrosta]

This API probably needs at least two "levels":

1. low-level GridFS helpers through the PyMongo object, for folks who want to roll their own
2. high-level view functions for accepting uploads and serving GridFS content (with conditional GET support)

The high-level API might look like this:

@app.route('/uploads/<path:filename>')
mongo.serve_gridfs(collection=uploads)

where serve_gridfs returns a view function that expects a single "filename" argument. Need to figure out how to set endpoint name so that url_for works as expected/as configured.

[dcrosta]

Turns out you can't use decorators on function calls, even if that function call returns a function. In any event, it's probably best for now to only provide the "low-level" helpers.