Merge pull request #81 from mgage/release/2.7

Fix off by one bug in number of attempts
Merge pull request #82 from mgage/release/2.7

Fix problem-content width=90% bug; Fix disappearing hidden sets.
Define WEBWORK_DIRECTORY if it is not already defined.

Backported from develop
Fixes issue #16
Merge branch 'release/2.7' of https://github.com/openwebwork/webwork2 into release/2.7