Refactored the tool to make it version-aware and more versatile
* ValidatoinProblems have a level and a category
* Categories cover most common problems found
* Output can be made more terse by reducing it to the category and property in question (--terse)
* More in-depth validation:
  * Required Properties
  * Recommended Properties
  * Literal Prop range checks
  * Object Prop range checks
  * Functional prop checks (non-repeatable)
  * Conditional required properties (e.g. if is-a dm2e:Page require bibo:number)
* Data Model version Aware: Must be selected on the command line
* Adapted NS.java to handle the new namespace (ugly duplication cannot be avoided
* Global validation strategy (i.e.: Go through all the Aggregations, CHOs, TimeSpans, WebResources instead of just following from the Aggregations)
* Unit tests