Adding the ability to set the application display name in AppRatingFlow

This would add the ability to set an application name other than the CFBundleDisplayName in the AppRatingFlow.  This could close issue #36

The reason that I created this is because my display name for my application is shortened to 'wyc' but I would like the "Do you love %@" to say "Would you choose" rather than the display name. 

This is just the way that I thought was the simplest solution to the problem.   
I have not tested this in any mac applications only my iphone application and the ApptentiveDemo project.

Below is how I use it in my application.

    ATAppRatingFlow *sharedFlow = [ATAppRatingFlow sharedRatingFlowWithAppID:@"x"];
    [sharedFlow setAppName:@"Would You Choose"];

Let me know if this doesn't make sense or you think it should be done a different way.
Cheers!
-Kevin