[WIP] Add test coverage scanner and CONTRIBUTING doc

I though it would be good to help new developers contribute back to `gulp-webserver` by using gulp to run tests and setting up a watch task to be able to run tests as they change. While setting up the gulp suite, I also added Istanbul to check test coverage.

The other piece of the PR is adding a CONTRIBUTING document. This will help to guide developers in a direction of how they can contribute to `gulp-webserver` using the gulp suite and the proper procedure of using branches and pull requests.