Make jobtypes versioned

This splits the jobtypes table into jobtypes and jobtype_versions, with
the latter containing all the actual data.

The version is a simple integer, to be incremented with every new
version.
Add validator for version, similar to max_batch
Adjust test to versioned jobtype model
Logic in validator more compact
Merge pull request #108 from guidow/jobtype_versioning

Jobtype versioning
Two minor typo fixes in docstrings
Merge pull request #109 from guidow/minor_typo_fixes

Two minor typo fixes in docstrings