CMS-2231 Create new data value type: ItemId

Created new class in package: com.enonic.wem.api.data.type

Created inner class for ItemId in JavaTypeConverter too (mimic ContentId).
Created inner class for ItemId in Value too (mimic ContentId).
Created inner class for ItemId in Property too (mimic ContentId).
Merge pull request #411 from enonic/CMS-2231

CMS-2231 Create new data value type: ItemId