Added UMD loading for AMD, Node, and globals

The goal of this PR is to allow users of requirejs or other AMD style loaders to depend upon reactive, with dependencies automatically pulled in. Without AMD or Node, it creates globals as before. I plan to add a new example app showing the use of this with requirejs, but wanted to solicit comments on the WIP, including:

   - Since it's most consistent with AMD or Node loaders to export *a single* object, I moved rxt to underneath rx and export only rx
   - Npm test will run grunt test *and* a smoke test that exercises the non-jQuery dependent parts of rx

I'll add the example app shortly...