Merge pull request #3 from McMartin/OnlyTypeAliases

Implement features as type aliases
Add TypeList specialization for empty template pack
Implement At<Pos> feature
Implement Front feature
Implement Back feature
Implement PushFront<Element> feature
Implement PushBack<Element> feature
Implement PopFront feature
Add InternalTypeList::InternalConcat
Implement PopBack feature