Allow users to specify the namespace separator

My company [Runscope](http://www.runscope.com) is probably going to use Flywheel pretty heavily as we expand our use of DynamoDB. We're already familiar with the basic ORM approach from our experience with SQLAlchemy. I personally built one project on top of Flywheel so far, and I've been pretty happy with it.

Anyway, we use table namespaces similar to what Flywheel supports, but we've already standardized on "." as the separator between elements. For example, our table names are `prod.atlas.hosts` and `test.scorekeeper.requests`. This commit allows Flywheel's namespace feature to work for us too.

This is my first pull request to the Flywheel project, so let me know if there are any stylistic norms that I should follow.