Adds custom titles to various pages in application

Fixes #121 

Logic in `_index.html.erb` feels a little wonky, but seems difficult to test in a helper method. Logic is tested in feature specs.

![screen shot 2014-07-02 at 10 35 15 am](https://cloud.githubusercontent.com/assets/1656824/3459647/4484d9ac-020f-11e4-850a-2f71a193b30f.png)

Maybe we can use a configuration/presenter here (although that can be a PITA under view tests).

I see we have one other reference directly to the `title_display` field in the collection masthead (and perhaps we should remove that at some point as well). This way if we ever decide that we want to use a different field for the main titles (which I think we actually may have to do for our production data) then we can just swap that in the controller configuration and all of our titles will be updated.