added EnamlTestAssistant working on Enaml 0.8

- added a TraitOperatorTestCase covering all of the trait operators for Enaml
- updated the GuiTestAssistant, TestAssistant and EnamlTestAssistant to work
  with Enaml 0.8
Merge branch 'fix-enaml-0.8' of github.com:enthought/traits-enaml into fix-enaml-0.8