put token to header if present

[kapucko]

Hi,

according to latest consul documentation for authentication: https://www.consul.io/api/index.html#authentication, consul client should insert token to the header when sending requests to agent.

This is my implementation, hopefully it'll help somebody.

Thank you for your great work.

Dominik

[kapucko]

@cablehead I tried to fix tests but there is still this issue:
ValueError: Plugin already registered: pytest_twisted=<module 'pytest_twisted' from '/home/travis/build/cablehead/python-consul/.tox/py27/lib/python2.7/site-packages/pytest_twisted.py'>

I have seen it also it other PRs. Could you help me please?
Thank you.

[tolmanam]

I took a very similar approach in my pull request (#205). I guess I should have checked here before submitting mine.
It's a bit encouraging that you also ran into issues with the unit tests.