Working with value instead of name, improving support for disabled attribute

[waza-ari]

Fixes #9, #10 and #11

Since the commits are modifying the same line, I had to combine the PRs for the three issues.

Unfortunately, I cannot get the tests to work in my OS X dev environment. Could be something related to the coding?

• values are now used instead of the display names to keep track of selected entries. Further, the values are returned instead of the names
• when selecting all items, the disabled attribute is now taken into account
• inverting now takes the disabled setting into account