update pylab messages

add `Populating namespace` message to `%pylab`

from @fperez
add `--matplotlib` flag to terminal IPython

just like pylab, but for `enable_matplotlib` instead
unicode fixes in SVG

minidom.parseString doesn't like unicode objects
re-run example notebooks without `%pylab`

prefer `%matplotlib` and explicit imports
update parallel example notebooks to remove `%pylab`
populate matplotlib / pylab enums from backends.keys()
move imports to %pylab description

from no-import-all description, which gets re-wrapped by argparse.
Merge pull request #3683 from minrk/matplotlib_magic

use `%matplotlib` in example notebooks

- removes any mention of `%pylab` in our examples
- adds `--matplotlib` command-line flag, mapping to the new
- adds `Populated namespace` message to %pylab output, at the request of @fperez.
- fix unicode bug in SVG, discovered during testing

closes #3680
closes #3684