added the editEnvironment method

Hi.  I'm a big fan of your library, and have been using it to avoid having to deal with knife/ruby ugliness with some of our deployment processes.  I've hit a minor road block as I need to be able to patch an Environment setting from one of my scripts, and the current version of chef-api does not have a method to do so.   This pull request adds the editEnvironment method, using the Chef-Api put method to update the environment.  I've tested the change against our own Chef environment and it works fine.

I would be much obliged if you'd be willing to merge this pull request and update your lib on npm.

Thanks a million,
-H