Removed 'ipython -pylab' references

This is a documentation fix for https://github.com/matplotlib/matplotlib/issues/3005. As referenced in the changelog, "ipython --pylab" is deprecated as of ipython v2.0, and will be removed completely in v3.0. The preferred method to enter the pylab environment is to call "%pylab" from the ipython shell directly.

Also, a note on one of the changes:

- examples/user_interfaces.py
        This demo script has been disabled since 2010, but there was a bit of code at the bottom that made use of a -pylab flag. I just left a comment about the deprecated flag for future contributors, since this pull request is intended only for documentation changes.