[Add] jsoncpp 0.6.2.rc2

This library requires a bit of work after being pulled down from SVN, I've added this in the `pre_install` hook.

For a c++ library like this should it have `stdc++` as a required library? I see some podspecs declare this but my experience is that this can cause linker issues if using `libc++` elsewhere in the Xcode project?