third_party: Add TinyXML-2 files from git

v2: Tinyxml2 CMakeLists.txt added
v3: Removed unused tinyxml2 files
v4: Fixed tinyxml2 CMakeLists file
desurium: Replace all occurrences of tinyxml with tinyxml2 equivalents. Also change build system

v2: Add changes to make tinyxml2 compile and run
v3: fix compile errors on systems without tinyxml
v4: don't search for tinyxml2 on win32
v5: fix win32 build errors

fixup
third_party: Remove tinyxml
travis/readme/install_deps: travis2 changes

v2: ubuntu usedy by travis-ci doesn't know tinyxml2
Merge pull request #598 from karolherbst/bug491

Replaces tinyxml with tinyxml2.