Added support for Auto Layout and creation from xibs

I've added support for Auto Layout by implementing `-instrinsicContentSize` on the control and slightly edited `-adjustFrame` to use AutoLayout when it's enabled.

I've also moved the initialization stuff from `-initWithLocation:emptyImage:solidImage:emptyColor:solidColor:andMaxRating:` into a common initialization method.

Calling this new initialization method from `-initWithCoder:` enables to create a rating control in a xib file.
As it's not possible to give a value to maxRating in the xib, I've added a public property `maxRating` public so it can be changed later (in `viewDidLoad` for example).