Merge remote-tracking branch 'origin/mathapp'

Conflicts:

mathapp.xcodeproj/project.xcworkspace/xcuserdata/Goda.xcuserdatad/UserIn
terfaceState.xcuserstate

merge mathapp branch into master
Merge branch 'master' of https://github.com/fdxyang/mathapp into mathapp

Conflicts:
	mathapp.xcodeproj/project.xcworkspace/xcuserdata/Goda.xcuserdatad/UserInterfaceState.xcuserstate