Follow path overrides for node_modules

[Bajix]

The way in which StealJS resolves packages is somewhat atypical, and gets in the way of doing requires that aren't prefixed.

Generally, projects will have their dependencies in an assets/lib/src folder. If this folder is set as the baseURL, then doing something like require('foo') should load ./assets/foo.js unless explicitly mapped elsewhere. If this where in fact an install dependency, then the path would resolve to ./assets/node_modules/foo.js. The trick to making this work properly is then to override the path of node_modules, like such:

System.config({
  baseURL: "assets",
  paths: {
    "node_modules": "../node_modules"  
  }
});

This would also remove the need for the #277, as packages would resolve to the proper paths without any additional syntax.