General Clean Up

This does some basic clean up, such as adding a `.gitignore` and a slightly improved `README`, restructuring the repository into something slightly less cluttered, and cleaning up the `CartoTweets` script. It also changes said script to read the authentication bits in from a YAML file. If you think that _which_ YAML file it reads credentials in from should be configurable, I can throw that together really quick with the `optimist` package.