tinyxml: 2.6.2 with debian patches and CMakeLists.txt

This formula will build a patched version of tinyxml 2.6.2
using the patches from debian. I've summarized the debian
patches in comments in the formula. Here's a link to the
debian patch management page:
http://patch-tracker.debian.org/package/tinyxml/2.6.2-2

I've also added a patch with a CMakeLists.txt that builds
a shared library and provides an install target.