add new feature: face center crop

__New Feature__

Picasso is built-in with two types of cropping:  `centerInside()` and `centerCrop()`.  They work well for most images, but sometimes they can cut out faces that are not located in the center of images, producing undesirable thumbnails.

With the help of the native [`FaceDetector`](http://developer.android.com/reference/android/media/FaceDetector.html) in the Android framework, this pull request adds a new API to do what I call a `faceCenterCrop()`.  It crops an image so that faces detected in the image are kept intact as much as possible.  Like `centerCrop()` and `centerInside()`, the `resize()` API must be called together with `faceCenterCrop()`.  For instance,

```java
Picasso.with(context)
       .load(url)
       .resize(targetWidth, targetHeight)
       .faceCenterCrop()
       .into(imageView)
```

However, the native `FaceDetector` API provided by the Android framework is slow.  You may provide your own implementation of the `FaceDetector`:

```java
Picasso.with(context)
       .load(url)
       .resize(targetWidth, targetHeight)
       .faceCenterCrop(new MyFaceDetector())
       .into(imageView)
```
oh! I thought it might be something that's global to the Picasso instance. 
Maybe it's better to set the face detector in `Picasso.Builder` rather than request level.