Mustache deferreds

[retro]

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?

[daffl]

This has been implemented in 2.2, closing this PR.