add awreece theme

I'm submitting this theme for possible inclusion in the repository.

It differs from most themes in the following ways:
-    Explicitly not having git or other version control info in the prompt (I very rarely don't know what branch I am in and when I care about the status of git, I usually care about which files are affected).
-    Showing the execution time and status of the last command executed.
-    On Mac OSX, displaying a notification if a (long running) command completes and the terminal isn't in the foreground.

Preview below (using Solarized Light colors):
![awreece theme preview](http://codearcana.com/images/zsh_theme.png)
![awreece theme preview](http://codearcana.com/images/zsh_theme_popup.png)

Some things that still puzzle me: 
-    I try to track the exit status of the last command, but the only way I know to do this is to record `$?` very early in the `precmd` (see line 96). Unfortunately, I'm pretty sure this would break if another `precmd` hook were executed before mine.
-    I cannot get the preview to display a right hand prompt.
-    I am not sure if it is completely ok that I am exporting `last_run_time` etc.