Memory leak fix

Hi,

I noticed that running the demo app with the iOS 5.1 simulator triggered the message:

```objc[33235]: Object 0x6823520 of class __NSCFDictionary autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug```

which appeared to be caused by the line:

```colorNameCache = @{ }.mutableCopy;```
  
in '+ (void)load' method of 'UIColor+MGExpanded.m'

I first tried fixing it by enclosing the line in an @autoreleasepool but after reading the answer to:

  http://stackoverflow.com/questions/10003962/breakpoint-pointing-out-objc-autoreleasenopool

I found that simply replacing the convenience method when initialising colorNameCache with an alloc/init was enough to avoid the problem.

Regards,

Paul.