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

See changelog.

This should, in theory, fix https://github.com/plone/plone.app.dexterity/issues/49

It also should enable folks who want to serialize and parse schemas containing Choice fields that have vocabulary terms with term.title != term.value.

Added numerous tests for ChoiceHandler (new) and additional doctest scenario for the new feature.