Add CORS support

https://github.com/fs/rails-base-api/issues/51

Before testing with `$.ajax` do:
```js
$.ajaxSetup({
    headers: { 'X-Requested-With' : 'XMLHttpRequest' }
});
```
what if we want to allow requests to more that one domain?

```ruby
def origins(*args,&blk)
          @origins = args.flatten.collect do |n|
            case n
            when Regexp, /^https?:\/\// then n
            when 'file://'              then n
            when '*'                    then @public_resources = true; n
            else                        ["http://#{n}", "https://#{n}"]
            end
          end.flatten
          @origins.push(blk) if blk
  end
```