Add :ssh-agent-options to open options

The :ssh-agent-options can be used to pass options to the ssh-agent
used for the connection.  Removes all logic around agent construction.
Add jump-host support

This changes the arguments for open to take a target sequence of hosts
to connect though.  Each element in the sequence is a map with :endpoint
and :credenttial keys.

Also changes to always use an in-memory agent, initialised with the keys
in the system agent.
Use RELEASE for lein-pallet-release
Updated project.clj, release notes and readme for 0.6.0
Updated version for next release cycle