LOGBACK-796 RollingFileAppender should have error when File matches FileNamePattern

#### Why

Ceki said:
> File property and the archived files should not point to the same
> file path. When you wish them to point to the same path, then File
> property should be omitted.

From http://mailman.qos.ch/pipermail/logback-user/2013-January/003655.html


I've added the code / test for this.

#### What
* I've logged the issue at http://jira.qos.ch/browse/LOGBACK-796
* I wasn't sure where to add the check, but I added it to `RollingFileAppender.start()` because that's where other checks where (as opposed to `RollingPolicyBase`, where it seemed like the checks should be)
* I wanted to make the least changes to the code, so I didn't want to add the concept of validation to file name patterns.
* The additional test went into `RollingFileAppenderTest`
* All tests still pass. :)

#### Other

Feedback, comments, etc. welcome.