eds LowLimit/HighLimit HEX signed/unsigned (CONTINUE)

[lamuguo]

In my understanding, 0xFF should be -1(int8), and 0xFFFFFFFF => -1(int32), too.

Would you double check the check here? https://github.com/christiansandberg/canopen/blob/master/test/test_eds.py#L60

If my understanding is correct, maybe we can modify the function: https://github.com/christiansandberg/canopen/blob/master/canopen/objectdictionary/eds.py#L214

A related issue here: #345