Better and simpler Python example

Aims of these changes are:

- simplify the example
- introduce the requests library http://docs.python-requests.org/, which is ubiquitous in Python ecosystem these days
- introduce Python2/Python3 compatibility (tested on both)
- fix minor issues with formatting and indentation
- remove unnecessary conversions of body payload according to Content-Type - we already get it as string, we can use it directly as string I suppose

This proposal should supersede both https://github.com/apiaryio/language-templates/tree/python-requests and https://github.com/apiaryio/language-templates/tree/python3 branches. It should also fix #5.

Also fixes https://trello.com/c/Zp06pT75/133-strange-indentation-in-python-examples-in-new-docs (internal).