Add .gitignore file
Ticket #216: Add RSH worker and make it a base class for SSH Worker.

rsh and ssh share a lot of common mechanisms. Code required for rsh is
very similar to ssh. Worker Rsh was done moving a lot of Worker Ssh code
into it. Worker Ssh now inherits from this new worker, only overloading
a minimal set of methods used to build the ssh and scp command line. All
other aspects are identical.

Closes #216
Task.py: Add a 'worker' default property for Task.

This property points to a class used when spawning workers through
Task.shell()/Task.run(). Default is Ssh.