Bipolar layer, save functionality, git ignore

Bipolar cells can now update themselves based on cone and horizontal
activities, but the outputs don't look like what we want.  More testing
required.
Added a save functionality based on the pickle library.  Retinas can be
generated, stimulated, saved and loaded.  Save files are stored as .p,
and since these files are quite large, I've added "*.p" to the gitignore
file