Move collectors into Rails app

Originally, splitting out each of the collectors into separate gems seemed like a good idea. In practice, the disadvantages of this approach tend to outweigh any advantages.

"Practicality beats purity." So, let's move the collectors into the Rails app.

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For each collector, all code (and assets) will reside inside the Rails app at `lib/plugins/stratify/<collector-name>`. All test code will reside at `spec/lib/plugins/stratify/<collector-name>`.

Some benefits of this change:

- Code reloading during development. When you're developing a new collector (or changing an existing one), you don't have to restart the Rails app after each change.
- Increased approachability for new developers that might want to contribute collectors.
    - They won't need to publish a gem.
    - They won't need to figure out how to test the new gem in the Rails app during development.
    - They just add a directory for the new collector (at `lib/plugins/stratify/<collector-name>`), and get to work. They can easily see other collectors as examples. Once the core classes are implemented, the collector is automatically included in the Rails app.
- Elimination of boilerplate code (and rubygems configuration) shared by each collector gem.
- Easier resolution of dependency conflicts. The Rails app's Gemfile manages all dependencies. When you add/update a gem dependency for one collector, you immediately know whether that version is compatible with the other collectors.

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TODO:

- [x] stratify-twitter
- [x] stratify-foursquare
- [x] stratify-garmin
- [x] stratify-github
- [x] stratify-gowalla
- [x] stratify-instapaper
- [x] stratify-itunes
- [x] stratify-rhapsody
- [x] stratify-base