Update exec.js

The spec implies that the last argument of cordova.exec() should be optional.  However, if it is omitted, iOS emits a cryptic NSInvalidArgumentException.  This can be avoided if actionArgs defaults to an empty array.
CB-6007 Fix findCordovaPath() not working when path contains a query param
CB-5438 Remove test symlinks & fix some build errors on windows.
Move all deps into devDependencies.
Add back "grunt btest" command