ssh_client fixture, updated/expanded ssh utils

- Needs a change to credentials.yaml, see credentials.yaml.template
- Tests!
selenium fixture now supports navigation fixtures
Merge pull request #79 from seandst/distributed-testing-selenium-shenanigans

selenium fixture now supports navigation fixtures
Merge pull request #83 from seandst/distributed-testing-ssh-shenanigans

ssh_client fixture, updated/expanded ssh utils