Search feature

![Screen Shot 2013-03-19 at 6 41 12 PM](https://f.cloud.github.com/assets/1718372/278928/ca16a078-90ff-11e2-9fa0-c50feb994824.png)

It's basically a copy of django admin's search. You specify `search_fields` in the model view. The difference is, it doesn't support full text search (https://docs.djangoproject.com/en/dev/ref/models/querysets/#search), since mongodb just got that feature (http://blog.mongodb.org/post/45754637343/mongodb-2-4-released), so MongoEngine will take some time to follow.

Do you care about IE compatibility (haven't checked if the css is fine - probably not)?

I haven't tested SQLAlchemy yet.