Fix parsing of empty Choice term to u'', not None, which addresses a cause of https://github.com/plone/plone.app.dexterity/issues/49 -- also explicitly construct SimpleTerm instances for each Choice field element, instead of relying on zope.schema constructors to do so.  This ensures that all terms have non-None title attributes.  Added tests for ChoiceHandler serialization and parsing.
Support Choice fields with terms containing distinct title from value as option, while preserving backward-compatible round-trip for all Choice fields where title is not distinct from value.  Includes tests for serialize, parse/read.  Uses OrderedDict as a mechanism to re-use IDict field rendering semantics for output of Choice field values where terms have distinct titles (py2.6 compatibility should be possible via zope.schema>=4.1.0).
Merge pull request #7 from seanupton/master

Fix parsing of empty Choice term, explicit term construction, support terms with title!=value, new tests