Add in a timestamp root schema
Get schema by file
Change tests to use a `default_schema` object

- Still a bit crufty, but less crufty than dumping the whole thing each time
Move `get_schema` into a libs/schemas

- first of many glorious schema utils functions m8
Make the schema return object a bit more obvious and descriptive
Merge pull request #155 from alphagov/jasonschemas

Add base 'timestamp' schema to datasets model