Added static methods to get platform type and platform string

This allows for getting the platform string from the platform model or platform type as well as getting the platform type from the platform model. This is necessary if you don't have a UIDevice object but have the platform model and want to get the platform string.

\+ (NSUInteger) platformTypeForString:(NSString *)platform;
\+ (NSString *) platformStringForType:(NSUInteger)platformType;
\+ (NSString *) platformStringForPlatform:(NSString *)platform;