Make Shadows package configurable

Makes the package for the Shadows class configurable. If no package is specified, the Shadows class is not generated. This feature facilitates a few things:

1. It enables the processor to be run in a validate-only mode. This is useful for allowing the in-editor, on-the-fly validation as you perform incremental builds without clobbering the generated files (see issue #1366).
2. The ability to turn off the generation aspect means that it can be used by consumers of Robolectric to validate their own shadows.
3. It enables the consumers of Robolectric to generate their own Shadow helper classes in different packages that don't conflict with the core Robolectric one. Thus it could be used by a user of Robolectric to (eg) generate a global `reset()` method for their own custom shadows.