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.
@sjagoe: Nor do I! :smile: 

The only reason it is there is to speed up the build. The repo has only a bunch of built PySide wheels. The building PySide from source is needed as apt-get binaries don't give you PySide for Python 3.3 and 3.4. Building PySide takes about 30 mins. Not good for build times, especially as now the build can finish in 3 mins. So, compiling the wheels before hand and hosting them on somewhere (I took a Github repo for the fact that it gave me enough space to host the wheels, and getting them was a one liner.)

I would say, Enthought should host these wheels somewhere else as soon as possible. (Possibly in Grits)