Fix suboptimal !pip install experience

[willb]

As a notebook user, I may want to install new packages without building a new notebook image. Currently, !pip install is broken, since nbuser doesn't have write permissions into either the system-wide site-packages directory or the conda-specific one. !pip install --user will work but the user directory is not in the PYTHONPATH, meaning an extra line of boilerplate is necessary to add it to sys.path.

Both of these issues should probably be fixed! (See also #13)