Added ability to use Markdown documents as Credits file

Added ability to use Markdown documents as Credits file.

I changed the appCredits property.

    @property (copy) NSString *appCreditsFileName;
    @property (copy) NSString *appCreditsFileType;

I also removed the connection between the NSMutableString (which is deleted) and the TextView in IB.


The standard font is now set to Lucida Grande, as this fits more in the OSX UI