Modify my virtualenv tools

- Add a function to use 'env' as the default virtual environment dir name
- Deactivate the currently active environment before activating another
- Improve the command line prompt
  - Truncate the path when within the current virtualenv directory (thinking about the structure at http://timmyomahony.com/blog/2012/11/09/general-django-project-structure-or-folder-layout/)
  - Split out the text formatting into their own variables to make the prompt variable a little clearer
Add pysubl script for opening Python modules in Sublime Text