Python 3 compatibility and Travis integration for it.

This PR is for a Port of Envisage to Python 3 using the same, common code-base for Python 2 and Python 3.

Major Changes:
  - `envisage.safeweakref`: uses a backport of `weakref.WeakMethod` from Python 3.3 for methods.
  - `envisage.developer.code_browser`: uses `ast` module instead of compiler package.
  - `envisage.plugins.remote_editor.communication.util`: handles `bytes` objects in Python 3 correctly.

Other than that for Python 3, Handling of imports of removed or renamed standard library modules has been added. Also, relative imports have been used for in-package imports.
Yes, i'd suggest we use the github release mechanism to upload the wheels to a new release on https://github.com/enthought/pyside/releases repo (an admin of the repo needs to do that).