Version 2.6.1

Added

• Add WITH_PROFILING option to CMake config. When enabled, this sets the
  -fno-omit-frame-pointer compiler option.

Changed

• Massive speed improvements when building multipolygons.
• Uses (and includes) new version 1.3.0 of protozero library.
• Removed dependency on Boost Iterator for PBF writer.
• Example program osmium_area_test now uses cerr instead of cout for
  debug output.