Tell Zend to not parse messages.po files

Zend tries to load messages.po files at every call. Let's avoid this,
since the files to be loaded are called messages.mo, but usually
developers keep their uncompiled version in place.

Please note that since parsing messages.po fails, they are not cached, so
Zend tries to load them at every page call.
Ignore also hidden files/directories
Merge pull request #1716 from mlocati/ignore-message-po-files

Tell Zend to not parse messages.po files