Support partial indexes

MongoDB 3.2 adds support for partial indexes. PyMongo doesn't technically need any changes to support them but we should do two things:

• Update the docs for Collection.create_index and Collection.ensure_index to list the new option, partialFilterExpression
• Add a test to make sure the option is passed to the server properly when used (MongoDb 3.2+ only).

For the test we can port this from jstests:

https://github.com/mongodb/mongo/blob/master/jstests/core/index_partial_read_ops.js#L12-L37