Updated all docblock type hints of Closure to FQN for accuracy / IDE compatibility

When using an IDE that supports parameter type checking (e.g. PHPStorm, Netbeans, etc) if you don't specify the correct type hints in your docblocks then you get false errors being reported because it can't determine the class (see below).

![error](https://f.cloud.github.com/assets/1880478/300567/e5c4c114-959c-11e2-8af4-306e293f91b5.png)

I've performed a project wide search / replace on instances of 

    @param closure
    @param Closure
    @return closure

and replaced them with the Fully Qualified Name for the internal Closure class.

    @param \Closure
    @return \Closure

This should prevent the IDE complaining and makes the docblock more accurate.