Merge vnotebook into master to add VNotebook widget

[sbordeyne]

PR Details:

• Widget name: VNotebook
• Author: @RedFantom

Description

A Notebook with vertical tabs

Checklist

• [x] Widget in a separate file in the appropriate folder
• [x] Widget functions properly on both Windows and Linux
• [x] Widget code includes docstrings with parameter descriptions
• [x] Included an example file in /examples
• [x] Widget is covered by unitttests in /tests
• [x] Widget includes required assets files
• [x] Reference to widget in AUTHORS.md
• [x] Entry in sphinx documentation

[j4321]

I think the error in the python 3.4 test with Appveyor is because ttkthemes is used in the example_vnotebook.py and probably the older version of pip or setuptools does not recognize requirements properly so we should add it explicitly in .appveyor.yml. However I am reluctant to add ttkthemes as a dependency just for the sake of one example, we could use the 'clam' theme instead.