Fixes to select widget

* Work normally when widget is in display or hidden mode
* Use the normal z3c.form widget's logic for determining available options
fix test
don't try to decode values that are already unicode
fix conversion of sequence field values that are missing
try fixing test again
handle vocab factories that are functions rather than classes
Merge pull request #40 from plone/davisagli-selectwidget-fixes

Fixes to select widget