Fix PermGen OOME

*Why I did it?*
When running multiple tests with RoboSpecification,
perm size inreases drastically.

*How I did it?*
Large perm usage was due to creating separated classloader for every
specification. Create RobolectricClassLoader as static field,
initialize it only once reuse it among RoboSpecifications.
Merge pull request #2 from mgrzechocinski/master

Fix PermGen OOME (shoult still fix encapsulation)