Ember.Handlebars.precompile supports knownHelpers

`Handlebars.precompile` supports a second `options` argument. Sadly,
`Ember.Handlebars.precompile` did not; it simply defined its own
`options` and passed them in. This adds support for passing in that
second argument. Specifically, the compiling client can pass in
`knownHelpers` so that Handlebars won't use `helperMissing` to do all
helper lookups.
Merge pull request #29 from zendesk/james/precompile_known_helpers

Ember.Handlebars.precompile supports knownHelpers