Changing wording to be more specific

I've asked on irc://irc.freenode.net/python and the proper term is `item access`, as in the methods `__getitem__`, etc.
Adding clarification to note
Retain category information in AttributeInfo's of CategoricalComponents

Closes #484
Merge pull request #507 from ChrisBeaumont/484

Preserve CategoricalComponent information in custom viewers
Merge pull request #499 from yuvallanger/patch-1

Changing wording to be more specific
Docstrings
Contracts interface
Merge pull request #512 from ChrisBeaumont/contracts

Setup PyContracts interface
Fixes to support updates to Ginga canvas and glue z-order