Python 3 support and quite big refactoring

This is quite big refactoring, that introduces Python 3 support. Python 2 is
not supported.

List of changes:

- The main thing, project now works only on Python 3. I tested it on Python
  3.4.

- Refactored buildout configuration, now buildout.cfg always extends one of
  config/env/*.cfg environment configuration file. Each environment
  configuration file extends config/base.cfg. This refactoring make
  configuration more flexible.

- Removed node.js dependency. Node.js was used only for Bower. Now Bower is
  replaced by config/assets.cfg buildout configuration, where all assets are
  downloaded using hexagonit.recipe.download recipe.

- Replaced homophony with django-webtest. django-webtest is better supported
  and have nicer interface for testing. Also rewrote all tests to use webtest.

- Rewrote z2loader to be compatible with Python 3, replaced manual parsing with
  configparser.

- corneti.recipes.codeintel recipe was removed, because this recipe does not
  support Python 3. Created upstream issue [1] for this.

- Added help target to Makefile.

- Improved test coverage report by excluding settings and migrations using
  .coveragerc configuration file.

[1] https://github.com/fabiocorneti/corneti.recipes.codeintel/issues/8