Make date fields be datetime objects

[esonderegger]

Fixes #12

This PR makes the date fields of all the classes in this library be datetime objects and, perhaps more importantly, adds a tests.py file that tests all of these fields.

I looked into putting the parsing logic in utils.py as proposed in the issue. However, I ran into the problem that the OpenFEC API returns a number of formats for its various date fields. I considered adding a dependency on dateutil, but wanted to keep the list of dependencies to a minimum.

Also yield cls(**result) gets called in multiple places in utils.py. It seemed like less repetition to handle conversion from string to datetime in the constructor of every class than to iterate over every key of every result in multiple places in utils.py.

Update (6/15/2018) I just updated this PR to use a utility function for setting attributes of an an instance, provided an object of date fields, with the value being a format string that can be passed to datetime.strptime.

I'm definitely open to doing this another way, though. I'd love to hear your thoughts.