#SB-178: Create a class which would call all storage validators

- Implemented.
- Moved the validators to a separate package.
- Removed the RepositoryGroupValidator. At the moment it's doing nothing sensible.
- Added a VersionValidatorService which loads all the version validators using a ServiceLoader.
- Added an integration test that tries to deploy an artifact with a release version to a SNAPSHOT repository.
Merge pull request #3 from strongbox/SB-178

#SB-178: Create a class which would call all storage validators