Assert on numeric status code values.

This makes it clearer, as we all know that the HTTP status codes mean.
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
Reword validation for clarity
Extract validation of absolute path...

... so we can re-use it elsewhere
Add rendering_app to ContentItem
WIP route registration