JDK 6, JUnit Assumptions and Segregate Tests

Add support for running JUnit tests with alternative JDKs as well as using assumptions to selectively skip tests. See commit log messages for details.

This pull request incorporates a slightly refactored version of Rob's pull requests:

https://github.com/SpringSource/spring-framework/pull/200
https://github.com/SpringSource/spring-framework/pull/142