Introduce newFromNumericId as intermediate step to replace legacy ids

This was suggested by Daniel. Currently we do have many places in the
code where a numeric id is converted to an EntityId object in some
way. This method is supposed to be an intermediate step to get rid
of all numeric ids. If it's used (I have to do this in other patches
since it's used in other modules) it will make it a lot easier to
find and fix code that still relies on numeric ids.
Update EntityId.php
Update EntityId.php
Merge pull request #30 from wmde/from_numeric_id

Updated inline documentation of EntityId::newFromPrefixedId