Beta label for certain Finders

As we launch Finders which may be considered in Beta, we want a way to display this on a per finder basis. We set this in the metadata in Finder API, which gets sent the content store as part of the details hash.

This commit sets this on the Finder object and displays the beta label if this is true.

![screen shot 2014-11-04 at 11 47 09](https://cloud.githubusercontent.com/assets/449004/4899108/6a88786e-6418-11e4-9262-ba59ba30600c.png)

Though that requires adding a proxy method in `Finder` rather than making it an attr_reader like everything else, which means we'd special case it. I don't mind enough that it's not a predicate for that to be done, so I'm going to merge this :)