Update Gemfile for Rails 4

- Update Gemfile to mimic the defaults that you get with a new Rails 4
  app (minus things I'm not using, like turbolinks)
- Use the edge version of mongoid, since Mongoid 3 does not work with
  Rails 4
Update all generated code for Rails 4

Take all the code (and other content) that gets generated when you
create a brand new Rails 4 app, and apply it to Stratify. (Delete the
content that was specific to Rails 3.)
Add mongoid-paranoia gem

Mongoid::Paranoia was extracted into a separate gem in Mongoid 4. Since
Stratify::Activity uses Mongoid::Paranoia, we need that gem.
Regenerate mongoid.example.yml using Mongoid 4
Declare missing field in Stratify::Garmin::Activity

Missing field was detected by test failure after upgrading to Mongoid 4:

  stratify-garmin collects and stores data from Garmin Connect
   Failure/Error: collector.run
   Mongoid::Errors::UnknownAttribute:

     Problem:
       Attempted to set a value for 'elevation_gain_in_feet' which is
       not allowed on the model Stratify::Garmin::Activity.
     Summary:
       Without including Mongoid::Attributes::Dynamic in your model and
       the attribute does not already exist in the attributes hash,
       attempting to call
       Stratify::Garmin::Activity#elevation_gain_in_feet= for it is not
       allowed. This is also triggered by passing the attribute to any
       method that accepts an attributes hash, and is raised instead of
       getting a NoMethodError.
     Resolution:
       You can include Mongoid::Attributes::Dynamic if you expect to be
       writing values for undefined fields often.
Use spork-rails for spork compatibility with Rails 4
Fix CollectorsController to work w/ strong_parameters

- Update controller to play nicely with strong_parameters
- Update specs (and start moving away from low-lever controller specs
  toward higher-level feature specs)
Fix #dom_id deprecation warning

> DEPRECATION WARNING: Calling ActionController::RecordIdentifier.dom_id
> is deprecated and will be removed in Rails 4.1, please call using
> ActionView::RecordIdentifier instead.
Fix :confirm deprecation warning

> DEPRECATION WARNING: :confirm option is deprecated and will be removed
> from Rails 4.1. Use 'data: { confirm: 'Text' }' instead.
Fix whitespace
Regenerate Capfile for Rails 4

Regenerate Capfile using latest Capistrano (v2.15.5).
Resolve encoding error affecting deployments

Prior to this commit, I was getting the following error when running
`cap deploy`:

> ArgumentError: invalid byte sequence in US-ASCII
> An error occurred while installing mongoid-paranoia (0.1), and Bundler cannot
> continue.
> Make sure that `gem install mongoid-paranoia -v '0.1'` succeeds before bundling.

Credit to @ruckus for suggesting this fix [1]. :sparkles:

[1]: http://ruckus.tumblr.com/post/18613786601/bundler-install-error-argumenterror-invalid-byte
Teach config.assets.precompile about plugin images

Ensure that images in Stratify plugins are precompiled for production.
Merge pull request #5 from jasonrudolph/rails-4-0

Upgrade to Rails 4