Improved error messages
Added missing import
Added some additional tests to nail down number repreesentations
Added cases to do some basic checks about schema validation

Adding these cases actually uncovered a bug in the underlying JSON schema validation
code which is address by https://github.com/xeipuuv/gojsonschema/pull/38.
Removed simplejson parsing of JSON

I did this because simplejson calls 'UseNumber' before parsing.  This confuses the
underlying JSON schema checking.  So instead, I call the normal Go unmarshalling for
JSON and then I trick simplejson into converting that into a simplejson.Json
object (which then allows me to use the simplejson convenience API).