Featuremapper

It's now ready to be merged, all tests are passing. I've tried to split everything out in a way that makes sense:

* analysis/featureresponses.py now only contains the interface to FeatureMapper, i.e. pattern_present, pattern_response and the storage and metadata fns. It also imports everything in fmapper.core for compatibility.

* command/analysis.py now contains all the Topographica specific measurement commands, i.e. activity, weights, CoG etc. and imports everything from fmapper.commands

* command/pylabplot.py now contains plotting code, primarily PylabPlotCommands.

* plotting/plotgroup.py now contains all the plotgroup definitions and Subplotting.

We should come up with a sensible reorganization of everything at the next CSNG, especially since Jean-Luc wants to make changes to the plotting code.

I've also added a GUI option to select the color channel (Orientation, Direction etc.), which also works for CFs now.