Review gruntfile and document + update

[Gruntfile.js](https://github.com/intermine/imjs/blob/dev/Gruntfile.js) appears to have a number of undocumented tasks and a set of tasks that are commented out.

We need to do the following:

- [ ] Look at the commented out tests and either resurrect them or delete them. Why are they commented out?
- [ ] Explore what the tasks in the gruntfile all do, and create a markdown file describing the tasks and why/how one would use them.