Image Effectastic.

Because how awesome is this?

![Stamen Watercolor + Desaturate](https://f.cloud.github.com/assets/53660/100109/a0947cac-67ec-11e2-8c7f-4462d92fc614.png)

... and this:

![Bing Aerial + Gloom](https://f.cloud.github.com/assets/53660/100113/1f868370-67ed-11e2-86ff-792f157f0b8c.png)

... and this?

![Stamen Terrain + Vibrance + Gaussian Blur](https://f.cloud.github.com/assets/53660/100171/09c6bf0a-67f2-11e2-9f05-85497c2dfe90.png)

 * Store image effect definitions in NSUserDefaults, allowing user overrides and easier definition.
 * Image effects define an array of filters, allowing filter composition.
 * Add a "desaturate" image effect.
 * Use a new defaults key (selectedImageEffectId) since the old key was an integer.
 * Use an affine clamp before applying filters (so that gaussian blur and gloom type filters don't look awful).