Merge pull request #30 from RoPP/python

Just to make source code clearer
Finer error message for missing submodule
Clean up .gitignore list  [ci skip]
Submodules have their empty dir

even if uninitialized
Makefile has to be included in tarball
C++11 requires Visual Studio 2013 (12.0) or higher
Windows compatibility fixes
Fix regression on non-Windows
Version bump in docs
Test for distutils module
Add output_style option to build_directory()
Add --output-style/-s option to build_sass command
Make AppVeyor to install the package
Release 0.6.0
Version bump
Mention C++11 in docs

See also #31
Mention #25 in changelog  [ci skip]
Mention C++11 in docs/index.rst as well [ci skip]
Follow up libsass upstream 3.0.1
Add :upcommit: extlink role