Add files parameter to the build object

[skierkowski]

This new parameter should be an Array of Hashes. Each hash has three key/values, a id, name and a url. By default this array is empty. However, the API should allow updating these values.

The idea is that the build processor will upload the newly created file to /files with a random ID. Then the worker will update the /build/:id to add the id, URL, and name to those files.

[skierkowski]

Covered in #26