Don't reinvent git

Use Git to track changes, rather than [reinventing it](https://twitter.com/konklone/status/353960261398433793).

This not only reduces the complexity of the code, but provides for better storage and analysis of changes over time by leveraging Git's built in version tracking and diffing capabilities.

Additionally, now other users that fork the project can take advantage of past crawls or perform analysis of past changes.

Other changes:

* Fixes compatibility issue when Twillo was not configured by wrapping config in conditional block.
* Rather than listing gems in the readme, lets just go ahead and user bundler.
* Use `open-uri` rather then shelling out to `wget`