Port to Python 3.

Major Changes:

  - envisage.developer.code_browser now uses ast module instead of
    compiler package.
  - envisage.safeweakref now uses a backport of weakref.WeakMethod
    from Python 3.3.
  - envisage.plugins.remote_editor.communication.util now handles
    bytes objects in Python 3 correctly.
  - Handling of imports of removed or renamed standard library modules
    has been added.
[TRAVIS] Run tests with 'nosetest envisage' in != Py2.6.
[TRAVIS] Start using built PySide, add conditionals for Py 2.6
[TRAVIS] Add keywords!
[TRAVIS] Install Apptools from python3 branch of pradyun\apptools
[TRAVIS] Install more python3 branches of enthought's packages.
[TRAVIS] Reorder installing order.
[TRAVIS] Remove no longer used .travis_before_install file.
[TRAVIS] Update nose to overcome regression in nose 1.2.1.
[TRAVIS] Add CI for Python 3.4.