API Tests: Changes to status code validations and added jsonschema validation

Moved most of the status code validations to the base class, in order to have them be more consistent across the individual tests.
Added jsonschema validation for the requests that return a json body
According to this:
http://en.wikipedia.org/wiki/Uuid#Variants_and_versions
those are the only values allowed: the hexadecimal N will always be 8, 9, A, or B

Just noticed that the maxLength I specified there is wrong. Should be 36, though the string maxLength for items that have a specific regex pattern probably doesn't do much.