Optional Kerberos dependency

[offa]

A first attempt to make the kerberos dependency optional (#349).

# default - no kerberos dependency
> pip install atlassian-python-api

# kerberos dependency included
> pip install "atlassian-python-api[kerberos]"

Open issues

• How to handle requirements.txt (separate file)?
• Update documentation (especially how to install extra dependencies).
• Are CI tests for both version available?
• Calling kerberos authentication without supplying the dependency should cause a failure.
• How to get both use cases into pip? (Or does it already work via setup.py?)