Version 2.6.0

Added

• The new handler osmium::handler::CheckOrder can be used to check that a
  file is properly ordered.
• Add new method to build OSM nodes, ways, relations, changesets, and areas
  in buffers that wraps the older Builder classes. The new code is much easier
  to use and very flexible. There is no documentation yet, but the tests in
  test/t/builder/test_attr.cpp can give you an idea how it works.
• Add util class to get memory usage of current process on Linux.

Changed

• New Buffer memory management speeds up Buffer use, because it doesn't clear
  the memory unnecessarily.

Fixed

• osmium::Box::extend() function now ignores invalid locations.
• Install of external library headers.
• Check way has at least one node before calling is_closed() in area
  assembler.
• Declaration/definition of some friend functions was in the wrong namespace.