pylab import adjustments

- add `%pylab --no-import`
  - allows override of the import_all option when the magic is called
- adjust `import_pylab` to import nothing when import_all is False

it's all or nothing now, not all or some. Now `pylab_import_all=False` should mean that *no* symbols are added to the user namespace.

This also removes `figsize` from the pyplot namespace, which was weird and confusing.

closes #2630
closes #2664
closes #3436