Proper class for RST dependency tree

This is a follow-up to kowey/educe#19 .
It introduces a class for RST dependency trees (`RstDepTree`), with two major differences with respect to the already existing RST structures:
* dependency trees are not defined recursively,
* this class inherits from `Standoff` but not from `nltk.tree.Tree`.

This new version should pass the unit tests, thus enabling successful build by travis CI.

I have also done some changes to improve conformance with pep8 and pylint.