Add uglify helper

I was upgrading from 0.3 to 0.4 grunt version in a js lib.
This js lib used a grunt's helper which has been depreciated:

`grunt.helper('uglify', content, options);`

I looked for the [uglify task](https://github.com/gruntjs/grunt-contrib-uglify), but I didn't see this helper there.
What you think to add the uglify helper into this package?

`helpers.uglify(content, options);`


Thanks.