Update ember_templates compilation

The ember template compilation output is currently written to the `app` folder which violates ye olde Yeoman rule to never change the `app` folder's content (and results in a dozens of jshint violations in subsequent grunt runs). 

Also, the compilation step was missing from the `build` meta-task, so templates weren't compiled before `usemin` was looking for them.

The compilation output is now written to `.tmp`, which works fine in server mode, but unfortunately requires a separate usemin build block until 2.0 enables a more flexible lookup configuration.