Merge branch 'release/3.1.0'
Merge branch 'release/3.1.1'
Merge branch 'release/3.1.2'
Merge branch 'develop'
Merge branch 'develop'
Natsort version 3.3.0 release.

This release provides the following updates:
    - Added a 'versorted' method for more convenient sorting of versions.
    - Updated command-line tool --number_type option with 'version' and
      'ver' to make it more clear how to sort version numbers.
    - Moved unit-testing mechanism from being docstring-based to actual
      unit tests in actual functions.
      - This has provided the ability determine the coverage of the unit
        tests (99%).
      - This also makes the pydoc documentation a bit more clear.
    - Made docstrings for public functions mirror the README API.
    - Connected natsort development to Travis-CI to help ensure quality
      releases.
The develop branch shows Travis-CI develop badge.