Updated source map to resolve #116

The source map feature of `uglify` has been known to be in need of work for quite some time. For even the most basic project structure, a [hacky fix](https://github.com/gruntjs/grunt-contrib-uglify/issues/39#issuecomment-14856100) is needed to get the source map functioning properly.

This is one solution that differs from solutions such as the request in #116 in that it breaks backwards compatibility.

This solution is as simple as it gets, I think, mimicking `grunt-contrib-sass`'s implementation of source maps by having just a single Boolean that determines whether you get your map or not. I believe this would be more useful to people than perpetuating the complex series of options required to generate a working source map currently.

`grunt-contrib-less` has an even more customizable source map implementation, but there are a number of open bugs about source maps in that repository right now, which is why I went for the simpler system.

There are new tests and an updated `Readme`, but the `docs` section wasn't modified.