Added missing constants for PXI and USB
Added with_metaclass compatibility helper
Added attributes classes
Refactored ctwrapper code to use pyvisa.attributes
Changed ctwrapper relative imports to absolute as other backends
Added some of the missing attributes
Using attributes in resources.
Work in the documentation
Changed intersphinx conf to the new format
Changes to the api part of the docs
Improved the way the values api is exposed
Documentation related changes in the code