Added ability to change button colors via Core Graphics.

#### This pull will not affect any apps currently using SIAlertView. It adds opt-in features.

That said, this is a relatively big change so don't feel obligated to merge it if you don't want to :)

I wanted to change some of the characteristics (color, shadow, etc) of the buttons but I noticed they were images. I tried to replicate them a closely and as I could using core graphics so the background, shadow, and virtually any other property can be easily changed.

By default, SIAlertView still uses the images. However, if you want to customize the buttons you can use UIAppearance like so:

```obj-c
[[SIAlertView appearance] setUseCoreGraphics:1];
[[SIAlertView appearance] setDefaultButtonColor:[UIColor colorWithRed:18.0f / 255.0f
                                                                    green:138.0f / 255.0f
                                                                     blue:177.0f / 255.0f
                                                                    alpha:1.0]];
[[SIAlertView appearance] setDefaultButtonFontColor:[UIColor whiteColor]];
[[SIAlertView appearance] setDestructiveButtonColor:[UIColor superBrightRedColor]];

// etc...
```

Here's a sample:
![ios simulator screen shot may 31 2013 7 17 54 pm](https://f.cloud.github.com/assets/564781/593937/c04d9602-ca4b-11e2-92a7-e88ef5a6d024.png)

I've only exposed a few properties but I think moving to Core Graphics could make SIAlertView really powerful. I'd love to continue adding to it after this.

Let me know your thoughts / concerns!