Merge pull request #254 from pstaabp/develop

pstaabp/Develop
updated OPL-update to include building library directory, subject and textbook trees.
Merge branch 'develop' of https://github.com/openwebwork/webwork2 into develop
Merge branch 'develop' into feature/new-homepage

# Via Peter Staab (2) and Michael Gage (1)
* develop:
  updated OPL-update to include building library directory, subject and textbook trees.