Revised the API for the `RAToast` and minor changes to `RAToastView`.

The `RAToastView`-class have to be initialized with the `RAToast` instance to
be able to access all of the toast related options.
Replaced custom animation methods with standard `UIView` animation.
Minor modification to the x calculation, also added rounding for calculations.
Added TODO note regarding the implementation of top and centered gravity.

Also, modified the sample application. The three toasts now use different
gravities, however not yet fully implemented.