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.
Merge branch 'readblock' into tipidev
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.
Make GPIBVisaDriver.raw_recv buffer smaller, add debug statement
Fixed error in VISA install_handler
Moved basic raw_recv implementation from GPIB to MessageVisaDriver

Because it also works for USBVisaDriver, which currently has no raw_recv.
Merge branch 'tipidev' of https://github.com/ignamv/lantz into tipidev

Conflicts:
	lantz/visa.py