review: more notes about shadow history

Merging branch: ~gael-varoquaux/ipython/ipython-sync-frontend.

Merging upstreat changes.

Updated the docs for using MPI with IPython.

Merging changes in branch ~vvatsa/ipython/ipython-trunk-merge.

looks like a typo in the exception log msg

Updating docs to reflect recent work on ipcluster. * Updated changes.txt with descriptions of recent work. * Updated development/index.txt to include new config blueprint. * Added note in ipcluster to say that this is a tech preview.

Final few cleanup changes to the PBS cluster in ipcluster.

More changes to the PBS batch cluster.

Initial version that works with PBS.