PythonInstalled: Don't require superenv

* The python do ... end block does no longer
  require 'superenv' but saves and restores
  the ENV by using ENV.to_hash and `ensure`.
  This should resolve some build problems
  with formulae using `env :std`!
* `python_helper` is now in a module `Python`.
* Imporoved some comments in python_dependency.rb
python: --HEAD option added and more robust builds

* Added a head for python 2.7
* Using `build.with?` instead of `build.include? 'with...'`
* More robust temporary linking of opt/python/bin during
  install and cleanup (with ensure).