Added Images to docs and fill gaps

[HHammond]

• This addresses https://github.com/HHammond/PrettyPandas/issues/17 as well as the fact that there are currently no examples to motivate people that this actually works or looks pretty.
• This PR should also try to make the Readme.md, Readme.txt and index.rst files look more similar. Currently they all display entirely different information.
• Readme.txt needs to find a way to link to the documentation.
• Readme.md should use screenshots from the docs/source/ folder