Add CORDOVA_HOME environment var

I thought it would be nice if the developer could set a fallback project path, in case (s)he's not working inside a cordova project tree. This is very useful when one is working on a project that has its main cordova dir as a sub-directory. Instead of cd'ing into the dir and calling the CLI from there, one can just set the environment variable. I see it as very useful for shell scripts, etc...

Please let me know whether you like it. I am available to make any changes needed.