Adds the httpRoot option to fix sourceMappingUrl relative paths in the footer

My current project is set up as follows:

```
Gruntfile.js
app/
----- assets/coffee
----- public/js
```

My coffee task looks like
```
files: [{
  expand: true,
  ext: '.js',
  cwd: 'app/assets/coffee',
  src: '**/*.coffee',
  dest: 'app/public/js'
}],
options: {
  sourceMap: true,
}
```

The generated js currently has a footer with
```
//# sourceMappingURL=../../../app/public/js/index.js.map
```
which is an invalid path, as my http root is app/.

This pull request creates a new option httpRootDir such that when it's set to 'app', the footer correctly reads
```
//# sourceMappingURL=../../public/js/index.js.map
```

Unit tested & manually tested with my own project.