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).

Closes https://github.com/dgeb/grunt-ember-templates/issues/54.