Retrieve unsorted events from Mongo

When performing an $or query in Mongo, if you specify a sort it cannot
use indexes. This led to some pretty severe performance implications.

Therefore, the sorting was moved in to code as generally this will be a
cheap operation, outweighed by the time saved by doing multiple queries
with one $or-based query.

http://docs.mongodb.org/manual/reference/operator/query/or/#or-and-sort-operations
Merge pull request #14 from adambird/no-sort-or

Retrieve unsorted events from Mongo