Adds hinting for list objects to the add method

[mlissner]

This change makes it so that the method docstring supports either lists or dicts. I ran into this issue when Intellij was unhappy because I was passing a list to the add method instead of a dict, as the method requested.

This fixes the linting issue in my IDE, and apparently, there's no standard on this (yet), so it should be a decent way to proceed. Here's the documentation for Intellij and PyCharm, where they describe the syntax that they prefer (they're usually pretty sane about these things):

https://www.jetbrains.com/help/pycharm/2016.1/type-hinting-in-pycharm.html#legacy

[coveralls]

Coverage remained the same at 95.369% when pulling d0c720e on mlissner:patch-1 into 6075730 on lugensa:master.

[mlissner]

This is a very small tweak to the type hints that should be ready to land.