Allow user to pass both - "password" and "key" argument to the
ParamikoSSHClient constructor. Before this change, those arguments were
mutally exclusive.

Part of LIBCLOUD-461, resolves #194.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Add a docstring which clariries how the auth works.
Specify True for look_for_keys and allow_agent option.
Update affected tests and add some news ones.
Some more docstring love.
For backward compatibility, only specify look_for_keys and allow_agent option
if "password" and "key" arguments are not provided.