Use handlebars and ember-template-compiler npm packages.

This allows easier management of the internal `handlebars.js` and
`ember-template-compiler.js` depencencies by deferring to their respective npm
packages.

The `ember-template-compiler` package will be updated as part of the standard
Ember release process (and is currently matching 1.2.0's compiler).

The previously, added options (`handelbarsPath` and
`templateCompilerPath`) are still used if provided (maintaining backwards
compat).