Implement delete method for single jobtypes
Fixed some docstring mistakes
Implemented jobtype/code endpoint

This allows clients to directly fetch the python code of a jobtype.
Bugfix in single jobtype/get

For some reason, SQLAlchemy sometimes returns the code column of JobType
as bytes instead of a string.  If this happens, convert it to string.
Minor docstring fix
Implement PUT method for single jobtype/code