Adding a max image size to RZImageCache

This is helpful for decompressing images on an iPhone 4/4S or a device with low memory.  If images are coming in over 5000x5000 pixels it can lead to a lot of crashes when we create the context for the decompression.  This creates a smaller context and draws the image at a smaller size to it.

A possible use case would be:
```
    if (DeviceIsIphone4())
    {
        [[RZImageCache sharedCache] setMaxDecompressedImageSize:kIphone4MaxImageSize];
    }
```
@Raizlabs/maintainers-ios Can you take a look at this and make sure this works.