Add parallelCollectionScan helper

The parallelCollectionScan command - added in MongoDB 2.5.5 - opens a configurable number of cursors against the specified collection and returns their cursor ids. These cursors are used to iterate the entire collection using multiple threads concurrently.

In PyMongo the parallel_scan helper will return 'num_cursors' instances of CommandCursor that can be iterated concurrently by multiple threads (or greenlets, etc.).