Implement a rudimentary pull with nested arrays.
Add support for $pull with positional operator.
Missing pymongo import in tests for pymongo.errors.ConnectionFailure usage.
Add $push functionality.
Add positional operator for $inc. Also, fix an issue with multiple edits and positional operators.

The problem was that the document would change after the first edit command.
This would result in the spec not matching on the second edit command and
thus the second edit not succeeding.

So we now store the subdocument in the update method. If the subdocument is
found, we can simply reuse it.
There are cases in which the document can contain an array. Mongo handles it and so should the mock framework.
Fix $push and $pull to handle more cases.
Add more tests cases to test suite for $push, $pull, and $inc.
Merge remote-tracking branch 'upstream/master' into pull-nested-arrays
Fix issue where we need to use iterkeys from six.
Revert imports to what they were before.
Refactor the subdocument searching into its own method for reusability.
Add more doc. Also used to kick off a new build on Travis CI.
Modify README to kick off tests.
Merge pull request #47 from abc2mit/pull-nested-arrays

Implement $pull, $push, $inc with nested arrays.