replace underscores in template filenames with dashes

When I tried to use the select widget in my app, I got the following error message:
```
Uncaught Error: Assertion Failed: You specified the layoutName select_item_layout for <(subclass of Ember.Widgets.SelectOptionView):ember1641>, but it did not exist.
```
I'm using Ember 1.7.0-beta.4, but I was able to reproduce this bug in 1.5 and 1.6 also.

I tracked down the problem to the Ember resolver normalizing all the underscores into dashes. So Ember.TEMPLATES.select_item_layout exists, but the resolver is looking for 'template:select-item-layout'.  I don't know if there's a better way to deal with templates that have dashes in their filenames, but the workaround of just renaming the templates to use dashes worked for me.