Don't require rspec/autorun

This is unneccessary unless you plan to run spec files directly instead
of via rake or rspec and also creates issues with test runners like
zeus because it adds an at_exit hook to execute specs.
Pass through options on as_json override...

... Otherwise much of the utility of as_json is lost.
Assert on numeric status code values.

This makes it clearer, as we all know that the HTTP status codes mean.
Whitelist publicly visible attributes

Safer than blacklisting the internal ones
Merge pull request #15 from alphagov/tidy_specs

Assert on numeric status code values in tests.
Merge pull request #16 from alphagov/minor-tweaks

Minor tweaks
Add a README
Remove id from example json output.

There is not currently a need for this field.  We can add it back if a
need arises.
Merge pull request #17 from alphagov/add_readme

Add a README
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