Update src/T4/OrmLite.Core.ttinclude

The following changes where made:
- changed ProperPropertyType to use CheckNullable function
- changed function CheckNullable to static
- changed function LoadTables to accept makeSingular param
  if makeSingular is false ClassName = CleanName so ignores Inflector.MakeSingular
- changed function GetPK to return "" if primary key colums is multiple columns