Make TransparentCompressionFilter work with newer jetty 8 versions.

The internal API for AbstractCompressionStream has changed in later jetty versions.
This makes the filter compile again under jetty 8.1.13, as there are no tests, this is strictly as
"compiles again" fix.

Also some PMD fixes to the code itself.
PMD fixes.
Merge pull request #18 from hgschmie/fixes

Various fixes