Draft ipcluster plugin refactoring to drop SGE requirement.

The NFS partition is used instead to be able to run:

    ipcluster engines -n numproc

on each non-master node of the cluster.
add timeout when fetching json file + simpler cluster shutdown
Use the cluster thread pool to start the engines on the nodes
Trying to open missing port for the client
Better startup message + be more robust to JSON changes
Make it possible to configure a custom notebook folder
Make it possible to restart engine, better logs, make msgpack setup explicit
Better docstrings for the stop and restart plugins
Open the controller ports for IPython 0.13.1
Update the documentation according to tests done on 0.13.1
Add section on engine restart + note on growing new engines
Better packer configuration support + updated doc
Revert unwanted change + typo
volume: fix bug when cleaning up failed volumes

Set self._volume = None after cleaning up a failed volume instead of
deleting the self._volume attribute altogether.
DOC fixes
sshutils: fix AttributeErrors in SSHGlob class

self.paramiko -> self.ssh
ipcluster: pep8 clean-up
ipcluster: raise exception if IPython isnt installed
ipcluster: newline cleanup
ipcluster: move inner function to class