libviennacl: Introduced ViennaCLInt as main integer type for OpenMP and OpenCL, not using any size_t anymore

Since Fortran has issues with unsigned integers, size_t will sooner or later cause troubles.
For flexibility, the integer type is now typedef'd to ViennaCLInt, so one can quickly change the
main integer type if needed.
The choice of 'int' was motivated by CUBLAS, where functions also take 'int' as integer parameter type.