Consider exposing urdfdom_headers version in a header file

[jslee02]

It seems the API changes introduced by version 1.0 are relatively small; switching to use C++11 instead of boost. If so, consumer libraries (like DART) could support for version 1.0 and other lesser versions with some preprocessors depending on the versions.

One issue of doing this is that urdfdom_headers doesn't expose the version numbers in the header files so that consumer libraries have to extract the version numbers from CMake variables. I propose exposing urdfdom_headers version in a header file, or it would nicer if anyone knows better way to get the version numbers in consumer libraries.