ADD: CMake support

Hello,

First of all, thanks for your great library. Manipulating JSON data as std::map is IMO the way to go.

I add support for CMake build system in order to try to ease multi platform support.
I have validated CMake generation and compilation with:
 * Visual Studio 2013
 * Clang (>= 3.4)
 * GCC (>= 4.7)

CMake generate an error when trying to generate with compiler that do not support C++11:
 * GCC < 4.7
 * Visual Studio < 2013

Hope it help