Fixes #206 by adding a new optimization pass that deletes unused fields.

1. add new file for optimizers in mongodb; create pipelinezipper and begin work on optimization to delete unused fields; prepare to move old optimizers into new package
2. delete old optimizer now that it's been relocated to new optimize package
3. got optimizer working for planner test cases, with updating test cases to reflect new behavior
4. add test case that was not passing before most recent optimization pass