Remove sortSlices()

This may be implemented in a future release.
Introduce new interface for output functions

This is another change that is designed to follow the DRY principle. The
_output() method of PieChart should be overriden by the child class that
is extending it. This way, we only need to write the headers in one
place.
Write output functions for PieChartGD

The PieChartGD class now supports GIF, JPEG and PNG formats.
Write output functions for PieChartImagick

The PieChartImagick class now supports GIF, JPEG and PNG formats.
Remove PHP 5.4 dependency

The nice new concise array declaration syntax, e.g. [1, 2, 3], requires
PHP 5.4. PieChart v1.* should support PHP 5.3 as it is, unfortunately,
still ubiquitous.