Replace Makefile with GruntJS

## Pull Request notes

This replaces [my previous PR](https://github.com/twitter/bootstrap/pull/7780), as requested by @mdo .

I have replaced the makefile with GruntJS. It supports all the previous commands and more. This should make things a bit more flexible and easily accessible for others to contribute too.

You still install everything with `npm install`.

I'm not an expert on this and there are a few things I was wondering about, so I'd like someone that knows more about what to expect from the build tasks to try this out. Let me know if you face any issues and I'll fix it.

## Commands

### Public, as per Readme:
`grunt` - takes the role of make, and does the basic package

`grunt dist` - concatenates and minifies files to /dist directory

`grunt test` - runs jshint and qunit

`grunt watch` - sets up a watch for LESS changes

### New, bonus:
All grunt tasks can be run separately. Such as `grunt jshint` etc.