Rewrite Net::Netconf::Access::ssh using Net::SSH2

This is a complete rewrite of Net::Netconf::Access::ssh, replacing all
prior use of Expect and the 'ssh' binary with Net::SSH2.

This means Net::Netconf no longer requires Expect (nor, by extension,
Tcl/Tk). It should also be much more robust, as the previous Expect
implementation is suspectible to breakage if the 'ssh' binary changes
its output.

Finally, it fixes issue #19.