GSIP 83 - REST Extensible and customizable versions report

First commit: geoserver get versions via ReST
second release
proposed relese
adding junit tests
introducing JUnit test
change rest mapping as requested
align tests to changes
adding documentation
remove geowebcache dependency from ManifestLoader;
remove duplicated Status codes section into rest config documentation;
improve rest-config-examples-curl documentation adding version and manifest examples;
Merge pull request #105 from ccancellieri/2.2.x-REST-manifest-version

[Backport] GSIP 83 - REST Extensible and customizable versions report