Sync enaml with chaco

This pr re-introduces the logic to overcome issue #168

With this `enaml-run` will behave as follows.

- If `ETS_TOOLKIT`  is set and the `--toolkit` option is not `enaml-run` will respect `ETS_TOOLKIT` value and use  the related backed.
- if `ETS_TOOLKIT` is set  and the `--toolkit` option is set to an incompatible backend then a warning is
raised, but `enaml-run` will still honor the `--toolkit` option.
- if `ETS_TOOLKIT` is not set then `enaml-run` with temporarily set the compatible backend in os.environ so that chaco and enaml components are in sync.