Add (currently disabled) tests for XML test cases using Reader.
Add comment.
Reorganized tests and added new one.
Use enum class instead of char for osc operations.
Refactoring of NodeRef and NodeRefList.

Now has its own iterators instead of using Collection. This should make the
iterator faster and it is now a bidirectional iterator instead of the forward
iterator it used to be.
Do not try to write out empty buffer.
Fix some warnings.
Create doc/structs.txt file on 'make doc'.
New exception osmium::unknown_type.
Suppress warnings from Catch unit test macros.
Change exception.