Headers

Minor compared to, say, issue #23 (proper ARC detection) but it would be useful to have header files copied to a sensible location instead of the old "Public Headers Folder Path" default of /usr/local/include.

See https://github.com/knellr/TBXML/commit/c6d54f03890aaf2f3814d13d6630d7f4cb03facd which uses the path "include/TBXML", allowing for TBXML to be included via `#include <TBXML/TBXML.h>`
