Adding `gulpplugin` keyword

Adding keyword so module gets added to http://gulpjs.com/plugins/
Tweaking package.json so it passes http://package-json-validator.com/ validator
Changing "gulpplugin" keyword to "gulpfriendly"

> Other libraries that are not file manipulating through streams but are made for use with gulp are tagged with the gulpfriendly keyword on npm.

Source: https://github.com/gulpjs/gulp/blob/master/docs/writing-a-plugin/README.md#about-streams
Merge pull request #6 from pdehaan/patch-1

Adding `gulpplugin` keyword