Added IEEE488.2 # format support to GPIBVisaDriver

Added function GPIBVisaDriver.read_block to read a block of data in
IEEE488.2 # format.
Support raw_recv'ing many bytes in GPIBVisaDriver

Added support for reading more than one byte at a time with
GPIBVisaDriver.raw_recv.
Check feat_key on WidgetMixin.on_feat_value_changed

Added a check to WidgetMixin.on_feat_value_changed so that widgets bound
to a DictFeat will only update when the value corresponding to their
feat_key changes.