proposed fix for #61 and #61 - Document.ready

Document.ready returns a promise.  Did not want to modify behavior of .ready? so as not to create any breaking changes, but perhaps to be consistent with HTTP.get that is what should happen.

Document.ready? now checks to make sure the Document is not already loaded.  It does not use $.isReady since this is not available in Zepto, but instead creates a variable to track this.  

Added test for the Document.ready promise

Updated Docs