Use the base_path as mongo's _id field.

Saves creating an extra unnecessary field and index.  Will also mean
that any foreign-key fields will contain more meaningful values.

This uses Mongoid's field aliasing[1] to hide most of the details of this,
hence no changes were needed outside the model.

[1] http://mongoid.org/en/mongoid/docs/documents.html#fields
Add unit-level test for as_json base_path
Merge pull request #14 from alphagov/base_path_as_id

Use the base_path as mongo's _id field.