Added handlebars-source gem generator to Rakefile

`rake gem` will generate a `handlebars-source` gem that ought to be used by third party handlebars consumers rather than them bundling their own handlebars.js

Usage:
`require 'handlebars/source'`
File.read Handlebars::Source.bundled_path
File.read Handlebars::Source.runtime_bundled_path