Make sure the TraitsUI control is destroyed then the Enaml object is.

I wasn't able to create a minimal failing example, but this caused a
segfault in a UI in a customer project.
PEP8
New method that collects all enaml widgets of a certain type.
Revised TraitsView to un-parent control before disposing.
Update dev_requirements.txt
Merge pull request #19 from enthought/fix-traits-view-destroy

Make sure the TraitsUI control is destroyed then the Enaml object is.
Merge pull request #23 from enthought/find-all-enaml-widgets

New test method that collects all enaml widgets of a certain type.
Merge branch 'master' into test-enable-canvas

Conflicts:
	dev_requirements.txt
Merge branch 'test-enable-canvas' of github.com:enthought/traits-enaml into test-enable-canvas