Add Binder to the project

[vezeli]

Changes proposed in this pull request:

• No changes to the API
• Add postBuild file to the repo which sets up docker session in Binder with matplotlib because windpowerlib doesn't come with matplotlib (this is now documented in README.rst)
• Add Binder badge and text in README.rst explaining how to run Binder (people that know about Binder will click on the badge on top)

This PR improves reproducibility of notebook examples. Binder is aimed for windpowerlib (and Python) beginners that would like to see how windpowerlib works in practice using the notebooks provided and don't want to install it locally.

To see how the README.rst with changes look like before merging, go to binder-example branch on my fork of windpowerlib. Start the jupyter session by clicking on the badge "launch binder" of the README page (or in the example section link) and jupyter page will open in a new tab in your browser.

Additionally, I removed problematic indentation in README.rst because it renders as quotation environment and now it renders as standard list.

[birgits]

Thanks alot @vezeli :) I will have a look at it later this week!

[birgits]

@vezeli this is great, thanks alot! I hadn't heard of binder before and I think it makes the first steps with a new tool so much easier :)

[vezeli]

No problems. Thanks for taking the time to consider the PR and thanks for providing windpowerlib module :)