Pawelkw's enhancements

Two pull requests from @pawelkw:
* [Fixed cropRect size being incorrectly calculated some aspect ratios when using PECropView.setCropAspectRatio](https://github.com/rivera-ernesto/PEPhotoCropEditor/pull/1).
* [The cropRect is now centered when PECropView.setCropAspectRatio is used to define a crop rect](https://github.com/rivera-ernesto/PEPhotoCropEditor/pull/2).