Python 3 compatibility and Travis integration for it.

This PR is for a Port of Envisage Python 3, to using the same, common code-base. 

Major Changes are in:
  - `envisage.safeweakref` now uses a backport of `weakref.WeakMethod` from Python 3.3 for methods.
  - `envisage.developer.code_browser` now uses `ast` module instead of compiler package.
  - `envisage.plugins.remote_editor.communication.util` now 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.