Added ability to run Robolectric tests offline

This pull request isn't totally production-ready yet, but I wanted to see if there is any interest in having this (or something similar) in the project.

Some of the Android files and other dependencies are typically downloaded using the Maven dependency resolution process at runtime. However, for my use case, our build machines are tightly controlled and hidden from the outside world. We needed a method to run Robolectric tests without requiring Maven. We also wanted to be able to check in the jars with the code so it's portable and doesn't require setting up offline Maven repositories on every build machine.

This is basically a port of some code from this discussion:
https://groups.google.com/forum/#!topic/robolectric/a8Pr_ChcWJs

You can set the following system properties to activate the behavior:
  robolectric.offline=true
  robolectric.dependency.dir=<path>

Runtime dependencies will be retrieved from the specified path instead of being downloaded from Maven.

Does anyone else see value in this, or is it too specific of a use case?