Many additions and improvements to installer.

- The app can actually run as a system service now, though it is still
  kind of jenky and prone to breakage.
- "Stages" renamed to "Tasks"
- New log function to simplify verbose output messages
- Now using set -e and heavy use of && for error handling
- New error function that simplifies error reporting
- Also a warning function to accompany it for non-fatal errors
- New download function to handle all downloading in a simple and
  uniform way
- Better checking for preexisting pip packages
- New task to create a system user for the dehr uwsgi vassal
- New task to create application directories and copy dehr into them
- uwsgi_emperor task now checks to ensure uwsgi is installed
- uwsgi_emperor task now creates the vassals directory
- nginx_vhost task now restarts the nginx server if a configuration was
  found to have already existed before creating the new configuration
- get_password removed; not needed
- All options and tasks are now documented