2.0.0

Drop Python2 support, change configuration filename and some minor improvements

• [Backward incompatible] Drop Python2 support (remove six, unicode litteral,
  update package configuration, etc..);
• [Backward incompatible] Changed default configuration filename from
  settings to boussole. This was required to avoid clash with some other
  projects since "settings" is a too common word;
• Drop pathtools package dependency since it is an abandoned project;
• Update doc config, use livereload and with new RTD config file;
• Drop support for libsass-python <0.19.4.
• Rename requirements_freeze.txt to frozen.txt;
• Add a new script for development which help to automatically update the
  frozen.txt file;

For new configuration filename change you have two way to resolve it:

• Just change your configuration filename to the new one, like if you were
  using settings.json, you will rename it to boussole.json;
• Use option --config to explicitely use your own configuration filename;