Load Grunt tasks automatically

Implements load-grunt-tasks to automate task loading.

- Reads the cf-grunt-config package.json to load its tasks.
- Loads project-specific tasks from the usual location.
- Documents the Gruntfile more extensively.

Dependent on cfpb/cf-grunt-config#3
Merge pull request #17 from Scotchester/slim-gruntfile

Load Grunt tasks automatically