Add built-in Handlebars helpers completion tests

Hey @fkorotkov, here's the tests I promised you.  These cover all the built-in Handlebars helpers.  These are pulled from the `Handlebars.registerHelper` helper calls in the Handlebars source ([for example](https://github.com/wycats/handlebars.js/blob/master/lib/handlebars/base.js#L100)).  I'll let you merge them in once you've got them passing.

Note that users (and libraries like Emberjs) can add custom helpers using `Handlebars.registerHelper` so potentially this can be replaced/enhanced in the future using Javascript references (thought it might be nice to leave the defaults in for Community Edition users).

Hope this helps!