Add CodeEditor datasource [DO NOT MERGE]

Created by: martinRenou

This data source allows the user to type python code or load an external python script which define his data source.

This branch must NOT be merged in force_bdss master. It will eventually be added to the Enthought plugins when those plugins will be extracted from this repository. It depends on traitsui and pyface, which shouldn't be the case of the force_bdss. This PR is only here for showing that some work has been done on a CodeEditor plugin, and that it should be moved later to an other repository.