Update installation instructions for cmi_plugins.
Remove the obsolete install_ext instructions.

The URL was wrong and install_ext is careless enough not to notice.
Including own URLs in source files is bound to get wrong soon.
Let's not do that.

install_ext is also bad choice for every updates in the cmi_exchange
repo would have to be installed separately.
Fix forgotten quote, join the second paragraph.
Avoid excessive GitHub links.
fix link name to nickel fit
Fix figure in-lining.
Ignore checkpoints from IPython notebook sessions.