Add add_padding() function to destructor of *ListBuilder classes

TagListBuilder, WayNodeListBuilder, RelationMemberListBuilder now call
add_padding() when they are destructed. This makes using those classes
a bit easier.
Use unique_ptr for *ListBuilder in XML input class.
Use unique_ptr for Node/Way/RelationBuilder in XML input

This removes the last explicit call to "delete". Yeah C++11!