cmake: use ${CMAKE_INCDIR_BASE} instead of hard-coding "include"
cmake: move all headers to include/opensubdiv/

The base include/ directory is too general for general
installation.  Specifically, "include/version.h" will likely
clash with many other projects.

Move headers to include/opensubdiv/ to prevent filesystem
namespace clashes.
Merge pull request #207 from davvid/header-paths

Looks like this is working - we will have to adjust some internal code when switching to this version, but this makes packaging much easier.

Thank you very much for this contribution !