Include preliminary support for text indexes

As the first pass at adding text index support to MongoEngine, here's a patch that lets you specify text indexes in the `Document.meta.indexes` field. To index a text field, prefix the field name with `$`, as in `$title`:

    class Book(Document):
        title = StringField()
        meta = {
            "indexes": ["$title"],
        }

If this looks like the right approach, let me know and I'll resubmit with a test case.

NB: Requires MongoDB 2.6.x.