Remove unused imports, variables
Use .tsv extension for cached Florida files

The files are tab-delimited, so make this clear from the extension.
This also makes it easier to open files by clicking on them.

Closes #88
Move generally useful methods to BaseLoader

Move methods from Maryland's base loader class to ``BaseLoader``.

In starting to think about the Florida loader, it's clear that
the functionality provided in these methods would be duplicated,
so it makes sense to just move them to the base class.

This makes the hook that should be implemented in state-specific
sublcasses the ``load()`` method rather than ``run()``.

Add docstrings to make the purpose of these methods more clear
and to help contributors understand how to use or implement the
methods in their subclasses.