Added custom type implementation test case
* the test case contained within the special use case tests implements a simple std::pair like type to map values
** it then instantiates a container specialized on this custom type and tests if custom types are basically supported
* this test cases also shows that some of the templates in the dtl namespace can be helpful to custom type implementations
** i.e. dtl::BasePointer and dtl::RelativePointer
** a helper template for defining the base type alias template could also be helpful