Search ancestors for node_modules in grunt.loadNpmTasks

Fixes #1211.

***Note***: the reporter of the issue suggested using `require`. I initially tried using that to implement this. However, I soon ran into issues, and I realized that there is a fundamental issue: a module can only be `require`d if it specifies a `main` file in its `package.json` ([docs reference](http://nodejs.org/api/modules.html#modules_folders_as_modules)). Many Grunt plugins do not do this--so that approach is out.