Update python setuptools and add pip

Created by: jcfr

Testing on Ubuntu 15.10.

Would appreciate feedback on Windows and MacOSX

This PR improves Slicer build system to support creating Slicer extension bundling python packages (e.g https://github.com/Radiomics/SlicerRadiomics/pull/13)

It teaches ExtensionManagerModel about extension bundling python packages.

It teaches SlicerBlockAdditionalLauncherSettings.cmake how to support custom launcher setting for generating AdditionalLauncherSettings.ini used by tests.

It adds the following variables to both the build system and SlicerConfig:

• PYTHON_STDLIB_SUBDIR
• PYTHON_SITE_PACKAGES_SUBDIR
• Slicer_USE_SYSTEM_python

It ensures AdditionalLauncherSettings.ini is configured only once. It will NOT be available anymore at the top-level for superbuild based extension. (this was an undesired side effect not a feature - and it caused the two files to be out of sync after introducing 420cfedf)

Packages updates:

• setuptools from 5.4.1 to v35.0.1 (latest version to date).

  • our custom patch for supporting visual studio are removed. setuptools now does a good monkeypatching job to fix the deficiencies of distutils
  • Note that we use a patched version supporting parallel build. See https://github.com/Slicer/setuptools/commit/2bea6fc523f0cf58dacab21c723f105c3652ecff

• include latest version of wheel and pip

• update from Python 2.7.11 to 2.7.13

Cc: @fedorov @pieper