Put problem edit form in tabs
Add ace editor to problem edit form

Used ui-ace
Implement an ace directive

unlike the uiAce directive, this directive can 
dynamically change the editor mode.
Use data-target instead of href for tab

href="./#targetId" works but data-target won't 
conflict for sure with angular router.
Add form error message

So far only report missing required fields
Merge pull request #29 from dinoboff/edit-problem

Put problem edit form in tabs