Develop a simple test suite

[thashepherd]

While we can test the results of this spider by manually examining search results and emails, I'd like something more automated. mail_sender.py and potentially demolition_spider.py are particularly ripe for testability, since they rely on environmental variables and/or arguments passed on the command line. Feel free to refactor existing code to make it more testable.

A great solution will include a way to automatically run the new test suite (likely using pytest) as a pre-requisite for making a commit.