fix shadowed variable problem.
Fixed multiple init problem. Also adopted modern singleton syntax.
Fixed implicit 'self' retain in block.
Fix implicit conversion issues with the NSUInteger.
The 'trackId' actually comes out of the JSON as an NSNumber, not an NSString. Don't assume it's a number nor a string, but check and Do The Right Thing™ in either case.
Merge pull request #14 from hsoi/fixing-compiler-warnings

Fixing compiler warnings