Ability to replace ClassLoader on test run

In my particular case Robolectric has to be launched for a long period of time, processing different classes (probably with the same names but different). So the way is to replace ClassLoader from time to time.
The method that actually replaces ClassLoader is located in ``RobolectricTestRunner``. To be able to get the current instance of ``RobolectricTestRunner``, new annotation was added.