Added caching to the datatype lookup to save database hits
Merge branch '91' into feature/cache

Conflicts:
	app/Umbraco/Umbraco.Archetype/PropertyConverters/ArchetypeValueConverter.cs

Fix conflicts and update caching to match #91
DRY up DataTypeId lookups/caching
Wrap method in GetCacheItem - missed from merge conflict in 3ffd03e34c175684891a38773c0d8ca98b8a0859