Add "e" symbol before virtualenv prompt segment
Update tmux theme and add sourceable config file

The tmux theme has been updated and now matches the other themes much
better. It may still look a little boring and segment cropping isn't
enabled, but it's a good starting point. It colors the weather segment
differently based on the current weather and temperature, and some other
segment colors have been tweaked.

To enable the theme, add the following to your tmux.conf:

    source '/full/path/to/powerline/bindings/tmux/powerline.conf'

Closes #51.