Adds Promises to Rye

This is my attempt to add [promises](http://wiki.commonjs.org/wiki/Promises/A) to Rye. It was inspired by jQuery's [Deferred object](http://api.jquery.com/category/deferred-object/). I tried to keep the API compatible with jQuery latest.

I realize that the code style is not the same as yours (mainly because I was used to mine and it read beter). If you want that changed I can push another commit to format it as such.

If promises is something that the Rye community would find useful (I know I would) then I could make another PR to convert the XHR parts of Rye to use a Deferred object internally like jQuery does. 

P.S. I'm curious why CoffeeScript was chosen for the tests but not used for the main source.