docs: richer documentation

* Adds information about changes, licence, and authors to the
  documentation.

* Adds Travis CI and Coveralls badges to the documentation.

* Cleans the wording of the tutorial, renames and reorders sections,
  removes unnecessary links.

* Fixes typo in `dictdiffer.patch()` docstring.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>