Renamed 'caller' utility to 'using' for more natural useage
Option to save a launch as an IPython notebook (if available)

If IPython is available, offer the option to save the launch
specifiers as an IPython notebook. Notebook path relative to the
current working directory or to the $LANCET_NB_DIR environment
variable (if set).
Cleaner, shorter pretty printing code with new 'annotation' option

Simpler and clearer pretty printing implementation. New annotate
option inserts an additional informative comment with the #items,