Update EnamlTaskPane to enaml 0.8.8 .
Update EnamlDockPane to Enaml 0.8.8 .
Update EnamlEditor to Enaml 0.8.8 .
Remove redundancies from EnamlTaskPane and EnamlEditor.
Small cleanups.
Add requirements for developers.

These are requirements for development tasks, as for example
running the tests.
Request relayout only if Enaml component supports it.
Reset the 'component' traits in the enaml panes after destruction.
Make enaml imports lazy.

This is to avoid that imports from pyface.tasks.api import enaml,
which is 1) undesirable and 2) breaks Canopy if it finds the new
PyFace while using the old Enaml.
Update the enaml-based example.
Merge pull request #92 from enthought/enaml-0.8-migration

Enaml 0.8 migration