Add option to store ops in single MongoDB collection

The current MongoDB storage stores ops in a separate collection for each document (see discussion in #115).

For our use, we're definitely going to hit the 12,000 collections per database limit, and using MongoDB's built in sharding seems like a much simpler option than creating multiple databases.

To use MongoDB's built in sharding, all of the ops need to be in a single collection. This pull request adds an `opsCollectionPerDoc` option to the Mongo storage.

(See also [Google Groups discussion](https://groups.google.com/forum/#!topic/sharejs/DIN2pNb6A3I).)