New Feature: Add Token

[slarrain]

This Pull Request adds a new functionality to the pydkan package: the ability to not only use username/password but also tokens.

The Good

• There is no difference in the way we use this package at all. Only at the beginning, when creating the instance, instead of using api = DatasetAPI(uri, user, password) we do api = DatasetAPI(uri, token) assuming token has already being defined.
• There were very small changes in some functions to allow this to work, but not on the "user space". The functionality is the same.
• The examples has been updated to use tokens.
• The README has been updated.

The Bad

• There is one thing that could break this implementation. Doing api = DatasetAPI(uri, token, True). If using token and want to set debug to True the way to do that is api = DatasetAPI(uri, token, debug=True).
• There is no token argument (at least with that name), so api = DatasetAPI(uri, token=token, debug=True) won't work.

The Missing

• I haven't updated the tests. If this PR gets accepted, I will definitely do that (and make another PR).