BEP-19: Plotting

Abstract: It would be nice to have a few functions to create publication-ready figures.
It seems to me that we currently duplicate a lot of code every time we need to create
our figures. Some of this is Brian-specific, but most is probably not.

Style options
-------------
* colors (BW default)
* figure size: possibly choices for different publications (with choice of 1 or 2 columns)
* fonts: could also be defined from publication styles
* point size (line thickness) and symbol size (cross, dot, etc)

General plotting functions
--------------------------
* add letters to panels (A, B, C...)
* axis scales with horizontal/vertical segments
* insets (e.g. zooms)
* ticks (not too many etc)

Brian-specific plotting functions
---------------------------------
* Add spikes to traces (uses a spike list and a state monitor)
* Remove reset lines from traces

Saving and restoring
--------------------
What would be nice is an elegant way to save a figure, or the data produced by
a script, and then make the figure in a separate script, with an easy way to
quickly redo a figure with a different format or new data.
Perhaps simply saving a dictionary of numpy arrays.
