GEOS-6533-6534 improve File/AbstractURL Publisher

- decode paths to correctly handle spaces or other encoded chars
- implement conditional get using If-Modified-Since headers
Merge pull request #614 from ischneider/GEOS-6533-6534

GEOS-6533-6534 improve File/AbstractURL Publisher
Merge branch 'master' of git://github.com/geoserver/geoserver