Fix minor grammatical error.
manage.py shell must be run in the virtualenv.

One of my group thought she needed to get out of her virtualenv to run
this command :-(
Merge pull request #200 from pozorvlak/master

Minor grammar fix; make it clear `manage.py shell` should be run in a virtualenv.
Made it clearer that runserver takes over the command prompt.

Thanks @pozorvlak for the report and original patch.
Explain requirements for a Heroku application name.