Add existingPackage to options for antdeploy task

This adds an option to the antdeploy task:

options.existingPackage
Type: Boolean Default value: false This option will tell the task to assume a package.xml file exists in the root folder. If this option is true the pkg data provided to the task will be ignored and a new package.xml file will not be generated. This allows you to reuse a package.xml file that may be present in your project.

Reason:

We have a project that combines some javascript/ui work with and apex classes, triggers, etc. in the same repository. This option allows us to keep the existing /src folder and /src/package.xml for use in our editor, while also providing the grunt deploy script.