Autoincrement fix with unit tests.  Float counter unit tests included.

This pull request includes three changes:

1. It fixes a bug where the increment parameter in the humbledb.helpers.auto_increment function is not used.  This fix makes the increment parameter operational.

2. It includes a unit test and setup code to test the increment fix.

3. It includes unit tests that expose a corner case where the sidecar document contains counters that are of type Double. The auto_increment helper function does not mask this behavior, and the unit tests are included to show this so other developers are aware that this can happen.
This reference to MongoClient is breaking tests on older verisons of PyMongo. See the Travis CI build error. You can use the _version submodule to toggle between old and newer client versions.