Make Token optional in NakadiClient class

[drummerwolli]

Is your feature request related to a problem? Please describe.
At least in the Zalando context, the access tokens expire. Usually they do that after 1-3 hours. The class objects can be set up for longer periods to get re-used and use the resources of the server more optimally. But currently the NakadiClient class takes as an input the token, which makes the class only live as long as the token is valid. This reduces flexibility when setting up the NakadiClient class.

Describe the solution you'd like
A NakadiClient class which does not require necessarily a Token to be passed and an option to pass the token which each call to a function (where necessary) to the class (e.g. post_event should accept a token parameter).

Describe alternatives you've considered
Set up the NakadiClient class on each request to Nakadi.