Load rabit library.

def _loadlib(lib='standard'):
    """Load rabit library."""
    global _LIB
    if _LIB is not None:
        warnings.warn('rabit.int call was ignored because it has'\
                          ' already been initialized', level=2)
        return
    if lib == 'standard':
        _LIB = ctypes.cdll.LoadLibrary(WRAPPER_PATH % '')
    elif lib == 'mock':
        _LIB = ctypes.cdll.LoadLibrary(WRAPPER_PATH % '_mock')
    elif lib == 'mpi':
        _LIB = ctypes.cdll.LoadLibrary(WRAPPER_PATH % '_mpi')
    else:
        raise Exception('unknown rabit lib %s, can be standard, mock, mpi' % lib)
    _LIB.RabitGetRank.restype = ctypes.c_int
    _LIB.RabitGetWorldSize.restype = ctypes.c_int
    _LIB.RabitVersionNumber.restype = ctypes.c_int