Polished the CONTRIBUTING file.
SB-186: Springify the indexer

Extract the indexing related code into a separate module.
#SB-186: Springify the indexer

Added a dependency to the strongbox-testing-resources.
#SB-186: Springify the indexer

Moved tests from the strongbox-storage module to the strongbox-commons, as the related code which is being tested, has also already been moved to the commons.
#SB-186: Springify the indexer

- Bumped the version of maven-commons.
- Removed the dependency on aether. We don't need Aether at the moment at all.
#SB-186: Springify the indexer

Refactoring in order to Springify the indexer.
Merge branch 'master' of github.com:strongbox/strongbox

Conflicts:
	strongbox-parent/pom.xml