cmd/bosun: Sort numbered results.

Useful for tables in templates:

```
    <h3>CPU For Monitored Processes</h3>
     <table>
    <tr><th>Process</th><th>Avg CPU in past 10m</th></tr>
    {{range $r := (.EvalAll .Alert.Vars.win_process).Sort}}
      {{if eq $.Group.host $r.Group.host}} 
            <tr>
              <td>{{$r.Group.name}}</td>
```

![image](https://cloud.githubusercontent.com/assets/1692624/5463391/71590638-854a-11e4-839c-54a015d50915.png)

Can we choose a more descriptive name? Maybe "DescByValue" since it's a descending sort by value?