Handle nil tag for chunks. Add nil-phrases filter.

Also adds tests for the noun-phrase filters.

Fixes #31
Changed chunk-filter to ignore chunks with nil tags.
Merge branch 'master' of https://github.com/ikarth/clojure-opennlp
Revert "Changed chunk-filter to ignore chunks with nil tags."

This reverts commit f4f8b510ac7fc59aeb4836bd2476520ba73a44cd.