Figure out template partials with Underscore 1.2.x

[yhahn]

Our current system of using the this scope for handling template partials seems to be bust with underscore 1.2.2+ changes where this in the context of a template is no longer the templates object. We need a new approach or a way to repair the scope of our template functions back to the templates object.

If we do make a breaking change for this it should belong in 2.x with 1.x staying on underscore 1.x.

/cc @kkaefer @tmcw @willwhite

[kkaefer]

This is fixed in jashkenas/underscore#372.

[yhahn]

Looks like it's made its way into Underscore 1.2.3. Just need to update, test, and release now.

[springmeyer]

I temporarily upgraded to underscore 1.3.3 since it transparently worked with all other apps - but it broke template rendering. Now finding this. Just tracking that I've reverted back to 1.1.x in 2a58afb

[springmeyer]

see also #43