Move stratify-twitter into Rails app

  - Move code from stratify-twitter to
    stratify-rails/lib/stratify/twitter
  - Configure Rails asset path to look for assets in
    lib/stratify/<collector-name>/assets/images
  - Configure RSpec to clean the database before any request specs
    (i.e., specs in spec/integration)
Move stratify-foursquare into Rails app
Auto-load collectors
Move stratify-garmin into Rails app
Move stratify-github into Rails app
Move stratify-gowalla into Rails app
Move stratify-instapaper into Rails app
Move stratify-itunes into Rails app
Move stratify-rhapsody into Rails app
Move stratify-base into Rails app (WIP)

This commit moves stratify-base into the Rails app, but there are a few
failing tests. It looks like we'll have to make some minor code changes
to allow the **full** suite of Stratify tests to run together.

The stratify-base test suite affects the content of
Collector.collector_classes, and that causes problems for tests
outside of the stratify-base test suite. One test in the test suite
should not have side effects observable by other tests in the test
suite. So, we've got a little work to do in order to remove the current
side effects.
Load core classes before loading any collectors
Require explicit registration of collector classes

As noted in ada607e, we had a few failing tests due to the stratify-base
test suite manipulating the content of Collector.collector_classes. This
commit fixes those failing tests.

Instead of using the .inherited hook to implicitly register collector
classes when a subclass of Stratify::Collector is defined, we now
require explicit registration of collector classes. By removing the
dependency on the inherited hook, we can register the collector
classes outside of the class definition lifecycle. We can clear the
registered classes whenever we like, and we can re-register them as
needed (which is handy for testing). This also means that we no longer
*require* that a collector class inherit from Stratify::Collector.
Move collector plugins into lib/plugins

  - Define the core Stratify classes directly in lib/stratify (as
    opposed to lib/stratify/base). With this change, the directory
    structure now matches the class/module structure
    (e.g., Stratify::Activity lives at lib/stratify/activity.rb).
  - Define all the collector plugins in lib/plugins, and add lib/plugins
    to the Rails load path.
Move Rails app the root of the repository
Remove obsolete capistrano extension

Now that the Rails app lives in the root of the repository, we no
longer need the capistrano-remote-cache-with-project-root gem.
Fix Rails autoloading for Stratify plugins

Customize inflections so that Rails can properly auto-load
Stratify::GitHub and Stratify::ITunes.
Auto-reload Stratify components in lib dir

Use #require_dependency instead of #require so that Rails will
automatically reload during development. Rapid iteration FTW!
Reduce explicit requires, in favor of auto-loading

Rails auto-loading will find the various activity types when they're
needed.

We still need to explicitly load each collector, so that all collectors
get registered, so that they're available in the UI for creating new
collector instances.
Update README

Update the README to reflect that the collectors now reside in the
Rails app, and that the Rails app is now at the root of the repo.