Enable 'visualize' button for script fields in Discover

Closes #2201 in several ways. 

- Normalize visualize buttons. Gets rid of the red/yellow/green button in favor of calming dark blue button.
- Replaces them with a contextual tooltip and a warning count depending on the features of the field.
- Changes the filter buttons to use the new .filterable property.
- Allows visualization for fields that are not indexed, but are scripted.

![image](https://cloud.githubusercontent.com/assets/1250387/5368460/bb255ef6-7fc3-11e4-98e8-2f812757de2e.png)

When you have multiple warnings, they just get mashed together. Doing something like this makes it a little clearer...

```js
warnings = warnings.map(function (warning, i) {
            var output = '';
            if (warnings.length > 1) output += (i + 1) + ' - ';
            output += warning;
            return output;
          });
```

![screenshot 2014-12-10 11 56 14](https://cloud.githubusercontent.com/assets/404731/5381995/98ff3694-8063-11e4-93dd-d1158163bfbe.png)