fixed bug #2399
added check expression for empty string
Merge branch 'master' into haoxuan2
Adding message from PM to fix bug number 2396.
fixed sticky button issue
fixed check equation box with unknown variable in student mode issue
Code review for "sticky" clicks and equations with
unknown variables.  Combined "toPM" and "cancelUpdate" to
mean that the student entry is neither sent to the PM or
saved in the model.
Merge branch 'haoxuan2'
Merge branch 'master' into bstrong
Code review for fix targe node strategy bug #2396.
Removed redundant hint line (this is why there were two
copies of the hints).
Updating Isle Royale problems to include preexisting nodes
Merge branch 'master' of https://github.com/bvds/LaitsV3
connection overlays for '-' and '/' code modified
Merge branch 'master' into pradeep
connection overlay addQuantity modified - tested for author mode
comments added for connection overlay code duplication
Fixed error in Author mode. Functions and Accumulators were causing errors when an expression was checked.
Renamed a test file.
Merge branch 'master' into pradeep

Conflicts:
	www/js/controller.js
	www/js/draw-model.js
Update wording from 'Author' to 'Group' in login.html and public-login.html.