Add proxy variables for flexible passing of data

Proxy variables will are now returned from calls to
functions with saveout=(...). These variables can
be passed to subsequent function calls and will
be transparently reconstituted on the MATLAB
side
run_code supports calling functions
Add util function for saving MATLAB figures

Allows one to do

with mlab.fig('foo.jpg') as f:
    mlab.plot(...)
Add examples of recent functionality