Improve json_handler decorator

It will now sanely respond with a 405 error if the method is not POST or
a 400 error if the input is invalid. A wrapped function can now indicate
by raising a certain exception that a status code other than 200 or 500
should be returned.
Fix typo in exception text
Merge pull request #213 from IONISx/fix-exception-typo

Fix typo in exception text
Correctly count the number of pylint violations
Fix pylint errors; dummy var regex
report total pylint violations regardless
Merge pull request #216 from singingwolfboy/db/fix-pylint-script

Correctly count the number of pylint violations
Merge pull request #211 from edx/gprice/improve-json-handler

Improve json_handler decorator
Add Set field

All hashable types are converted to sets.
Override the field constructor so that hashable types are converted to
set.
Add tests for Set field