Added batching to reduce wasted draw calls

Using the demo, I was able to save 50 draw calls by using the new batch method for updating the initial view's options.

```objective-c
-(void)batchUpdateViewWithInitials:(NSString *)initials circleColor:(UIColor *)circleColor textColor:(UIColor *)textColor font:(UIFont *)font;
```

You can test by putting an NSLog in the 
```objective-c
- (void)generateCachedVisualRepresentation {
NSLog(@"generateCachedVisualRepresentation");
//Rest of the code
}
```

Comparing the current version of BMInitialsPlaceholderView and the demo on the iPhone Simulator(3.5 Inch), to my version results in the following.

Current version:
60 logs of "generateCachedVisualRepresentation"

My version:
10 logs of "generateCachedVisualRepresentation"

This helped speed up my app dramatically.  Hope it helps someone else!   Thanks for the helpful component.

Cheers,
Ryan