Support for non-public @Parameter

Hello JUnit! 

This patch makes it possible to have @Parameter fields in @RunWith(Parameterized.class) tests non-public.

I'm not sure if my change to ParameterizedTestTest is really an appropriate and sufficient test for this minor extension. (But the patch does work and achieve what it should, of course.)

BTW: As I've also found that @Parameter was not yet documented on https://github.com/junit-team/junit/wiki/Parameterized-tests, I've taken the liberty of going ahead and directly adding a section about it (incl. ref. to this) - hope that's OK.

Regards,
Michael