Add support for client-side certificates

[larsborn]

This PR adds support for passing in the cert parameter to the session object (through the AikidoSession class).

I hope adding a new headline (with a new version number) to CHANGELOG.rst was the right move here. LMK if I should do this differently.

Limitations:

• I don't have time right how to understand grequests well enough to add this feature there as well. But I think throwing an exception in that case is better than not having client-side certificate support at all).
• I've reviewed the tests directory but don't really know where I would add a test for a feature like this. Would it be ok to merge this even though the CONTRIBUTING.md states that

If you add a new feature please provide a test for it. Otherwise your pull request might be rejected.

[tariqdaouda]

Merged it, thank you!