Fixed 64-bit issue by truncating the 4 higher order bytes of size_t which is converted to unsigned long on 64 bit machines.
This guarantees compatability between 32 and 64 bit architectures but comes along with some restrictions in usage:

- OpenMesh won't be able to store data of type unsigned long (if necessary split data into two uints)
- OpenMesh won't be able to store data types whose size exceeds the unsigned integer value range (which
is theoretically possible on 64 bit machines). Theoretically...

git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@127 fdac6126-5c0c-442c-9429-916003d36597