Mint.com single-page application workaround

Fixes #2 originally reported by @yebyen.

It appears as though Mint.com is moving more towards a single-page application, especially given the JavaScript included on [this page](https://wwws.mint.com/login.event): `spa-bootstrap.js`. To work around this, we'll need to switch to a JavaScript capable screen scraping tool.

This Pull Request builds on the quick and much appreciated contribution from @kingdonb in #3, with a significant difference: I propose we use [Poltergeist](https://github.com/jonleighton/poltergeist) instead of [capybara-webkit](https://github.com/thoughtbot/capybara-webkit) (please see 1bc7869 for rationale). I also took this opportunity to refactor the code into objects and remove the use of insecure command-line arguments (please see 058749b).

If you don't mind, please give this a try. I'd also appreciate any comments on the code. Thanks!