Add carrierwave and carrierwave-mongoid

We're tied to earlier versions due to carrierwave-mongoid's
dependency on Mongoid 3.0, which is in turn reliant on MRI 1.9.3
Create skeleton asset model

Validation that file is present - does not yet
handle file uploads.
Use Mocha for stubbing
Add asset uploader

Stock Carrierwave uploader, with the `store_dir`
and `cache_dir` paths changed
Test that an asset should be created

Stubbing out the file here - I'd prefer to test
the upload process on the Uploader itself.
Remove dummy route and spec
Add rabl gem
Set up test helper for Carrierwave

Use separate upload directory
Clean the directory after each test
Add assets controller with create method
Exclude uploads dir from Git
Add specs for creating an invalid asset
Add an asset factory
Add a 404 error response
Add a show action for assets

Right now, this just returns a recursive url for
the asset. This is intended to contain a file
access url and potential metadata in the future.
Expose the asset name and content type in response
Standardise response format across all responses

Use the `_response_info` hash, as used in content
api responses, to return status.
Add asset requests spec
Merge remote branch 'origin/asset-model'