Updated libgit2 usage for v0.21.0.
Removed Windows XP support from CMakeLists.

Also updated readme and build instructions.
Updated build instructions, changelog.

It seems that CMake 3.0.0's GUI sets the toolset to `vc120_xp` by
default, and it can't then be overridden in a CMakeLists.txt. Generating
using the command line is therefore more reliable.