Unescape topic title before setting it as the document title

Currently having a topic title with `<` in it will result in it being replaced with `&lt;` in the document title. 

Since the document title is set using `$('title').text(...)` and `document.title = ...` this is a safe change to make.

Example: http://try.discourse.org/t/topic-with-lt-in-the-title/190