Mustache deferreds

Implements `def` helper that can handle deferreds in the template code. 

Sample use:

	{{#def Todo.findAll}}
		{{#if isResolved}}
			{{@data}}
				... iterate over the data
			{{/}}
		{{/if}}

		{{#if isRejected}}
			{{@error}}
				... print out the error
			{{/}}
		{{/if}}

		{{#if isPending}}
			still pending
		{{/if}}
	{{/def}}

I prefer this syntax to anything other mentioned in the #179 because it's more explicit and it's immediately clear what's happening.

`def` helper can also handle non-deferred values in which case it will treat it as resolved.

@justinbmeyer any comments?