add `%matplotlib` and `shell.enable_matplotlib`...
add `%matplotlib` and `shell.enable_matplotlib`
Now `%matplotlib` / `enable_matplotlib` do the bulk of the work. `%pylab` / `enable_pylab` just add namespace population on top of that.
Related changes:
- remove `pylabtools.activate_pylab`
- shorten backend message to just `using matplotlib backend: <backend>`
- only show backend message when magics are called with `auto`,
never from Python methods or when backend is explicit.
- pylab clobber warning is only displayed in the magic, not the Python method.
