render templates server side for rails

using ExecJS and the handlebars javascript source
the handlebars templates is rendered on the server

objective is to keep templates DRY. same template
on the client and server

stole code from https://github.com/zohararad/handlebarer/blob/master/lib/handlebarer/renderer.rb#L24-L34
to introspect locals and controller variables
no need to pass in a special `handlebars` local like https://github.com/railsware/sht_rails

copied the api for https://github.com/pixeltrix/steering as well

thanks @zohararad and @pixeltrix :-)