SLING-3027 improve Launchpad Karaf features

- break down sling-karaf feature into more fine-grained features for better reusability 
- add descriptions and versions 
- add more features 
- update integration tests 
- update Pax Exam to 3.3.0 
- update Maven Failsafe Plugin to 2.16 
- add workaround for KARAF-1972 
- update README 
- cleanup

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1529429 13f79535-47bb-0310-9956-ffa450edef68