Adapt code to archive DMs

You know what's awesome? Patching a program, realising you should rebase your patch on the latest commit (I based off twitter-1.8.0, so had a fair few changes to make), and then finding all the features (namely Favourites and Mentions) that got added to master in the meantime! Love your project! I will likely try to tweak the Favourites and Mentions behaviours in the near future though so they and Timeline-fetching aren't mutually exclusive.

NOTE: You'd need to update your Twitter App settings to allow viewing and posting of DMs for this to work out of the box for people.

* Add argument to get DMs
* Adapt statuses_portion()
* Adapt statuses() to optionally handle DMs
* Adapt main() to pull down DMs if instructed