Use HTML and Localizable.strings to create the Credits text

In issue #11 I mentioned that "Transifex cannot handle RTF files so Credits.rtf cannot be used".

This change programatically builds up an HTML string, using `NSLocalizedString()` to translate, then constructs the NSAttributedString used by the NSTextView. 

This would make using Transifex possible for the credits screen. Only six strings need translating. Once done you would be able to delete the 17 Credits.rtf files.

There is a slight difference in the rendering, just with the bullet points:

![screen shot 2013-06-29 at 9 10 17 am](https://f.cloud.github.com/assets/1576190/725307/f06706fc-e058-11e2-8b6f-6708e5c86cde.png)

If you want to try this, then I'll need to get the strings up on Transifex and translated.