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.
Fix issue with $pipestatus

$pipestatus is a space-separated int list and not a single int.
Make g:powerline_* variables fixed*, added g:powerline_loaded

The latter is required to be able to forbid sourcing powerline.vim

* changes are ignored until script is resourced
Add installation of powerline.vim into /usr/share/vim/vimfiles/plugin
Revert to previous way of sourcing vim plugin

Refs #95.
Install source_plugin.vim in Arch Linux PKGBUILD
Update installation instructions for tmux and shell prompts