Removal of Mercury and Numeric version schemes, refactoring

[andrzejj0]

Removed Numeric and Mercury version schemes. Mercury is probably not used at all, Numeric can be used due to some artifacts, but since it is not respected by Maven Resolver itself, it makes little sense to keep it.

With the two comparators removed, we can simply use the default comparator that DefaultArtifact provides on its own (which is also in line with Resolver implementation of GenericVersion).

The rest is refactoring:

• Simplified code, added some unit tests while doing so;
• Extracted RuleService from VersionsHelper to handle rules-related business, e.g. ignored versions;
• Extracted ArtifactFactory;
• Renamed DefaultArtifactVersionCache to ArtifactVersionService;
• PropertyVersions: moved artifact resolution business to DefaultVersionsHelper;

[slawekjaranowski]

great, as it is next big change I will try to review it in next weekend
of course will be good if someone else also look at it

[andrzejj0]

Please feel free to invite other reviewers. Maybe Hervé or Tamas?