Set up Auth Flow

This PR sets up the model code to scan GDrive folders for ROMs and save off what we find (or not? Maybe it's faster to just scan the whole shebang every time). 

**Update:** This plan has changed like eight times though the course of the PR. Here's what we've found:

* Google Drive sucks too hard to live. Moving to Dropbox
* Trying to scan on the client is too damn slow. We need to create an AppEngine backend which can do the work for us.

### TODO

- [x] Set up our model code to describe our categories ("SNES", "Recent", "Favorites", "Newly Added")
- [x] Set up code to discover ROMs (i.e. recursively walk the GDrive folder)
- [ ] Rig the welcome page to show progress synchronously while we run through the folder for the first time (in the case that scanning GDrive is hella slow)