Update cmake config from the master in libosmium/cmake.
Improved cmake config.
Change cmake config to allow specifying C++ version to use.
Move essential compile options from OsmiumOptions.cmake to FindOsmium.cmake.
Add cmake config option to compile a specified handler.
Completely switch to CMake. Remove old Makefile build.

The CMake build is more flexible and supporting two different build systems is
too much effort. A wrapper Makefile is still provided for convenience.
Updated READMEs.