Begin working on improving test coverage

Add simplecov to Gemfile
Startup simplecov in spec_helper
Create empty specs where they didn't already exist
Add I18n config option to silence deprecation warning
Introduce nested routes for batch contributions

Any contribution can be associated with a batch and viewed as such, or
viewed as a general contribution in the all contributions list.

The nested routes will be handled by the BatchContributionsController,
while the non-nested versions will be handled by
ContributionsController.
Add BatchContributionsController and related views
Remove html and js responses/views from BatchesController

Instead of showing the contributions for a batch at /batches/:id/,
they are shown at /batches/:batch_id/contributions/.
Update contributions views to reflect partials no longer shared
Update batches/_batches partial to reflect batch contributions change
Change before_filter to before_action
Remove a few unnecessary lines
Add spec for ContributionsController and refactor a bit