WIP hacky fix for ember1.6-beta,1.7-canary see #29

we pass layout as a function to the show classMethod but we need to use
the string version by default

need more investigation to figure out what's happening
Merge pull request #39 from emberui/mattia/ember1.7-modal-fix

[WIP] fix for ember1.6-beta,1.7-canary see #29