Add more tmux segments

I've added some new tmux segments that I've found useful, inspired by [tmux-powerline](https://github.com/erikw/tmux-powerline).

Included are segments for user name, external IP, uptime, system load, weather, and dates/times.

Note: this relies on Lokaltog/powerline#41 to cache the results of weather/external IP, since they're too costly to refresh every time.

Here's what they look like:
![Screenshot from 2013-01-10 15:52:07](https://f.cloud.github.com/assets/77758/58169/9e6ad82c-5b67-11e2-999c-a4043c61f0aa.png)