Adds a header label to WPContentView.

If a the comment detail view is using a note content provider, add a header to the top that provides some context about the comment being viewed.

Looks like this (cc @drw158):
![ios simulator screen shot apr 2 2014 1 38 49 pm](https://cloud.githubusercontent.com/assets/789137/2597139/cb5c60be-baaf-11e3-92e8-b5e2356c56e5.png)


Fixes #1444.