Adds a preference for the classpath validator

The preference allows to disable the search for jars not compatible with the current version of Scala.
Limits the check to possible version of Scala >=2.8.0.

Fixes #1001482
Adds an extractor to check incompatible versions

The code is more self-contained.
Adds documentation to classpath validator.
Merge pull request #303 from skyluc/issue/classpath-validator-preference-1001482

Adds a preference for the classpath validator