Clean up project and delete unneeded files

This branch is a bit of an overhaul, so there was an immediate need to
clean everything up.

- Deleted Ubuntu and Yanone Kaffeesatz font families as they weren't
  being used, and added the licence for Open Sans.

- Renamed the example pie chart image, deleted its old directory and
  updated the reference in README.md.

- Deleted 'embed-example.php', 'gd/benchmark/index.php',
  'gd/download-test.php' and 'get.php'; as they were all half-finished
  ideas.
Create abstract class to reduce code duplication

The new abstract class 'PieChart' will define public methods and enforce
an interface for the different versions to extend. In this way, multiple
graphics libraries, such as GD, Cairo and Imagick, can be supported
without unnecessarily duplicating common functions.

- A consistent format for storing colours will need to be implemented,
  to avoid cross-compatibility problems between graphics libraries.

- The output and saving functions should probably be rewritten to ensure
  that headers are set and files are saved consistently; this will also
  reduce duplication.

- The code for the current Imagick and GD versions will obviously need to
  be refactored!
Create PieChartColor class

This new class will make it easier to handle colours in a library
-agnostic way. There is currently no need for transparency; so it
ignores the alpha component and treats all colours as opaque.
Refactor PieChart to use the PieChartColor class

See previous commit message for an explanation...
Refactor PieChartGD to use PieChartColor

- The method processColor() was deleted, as it is clearly no longer
  needed.

- The call to imageRectangle(), which drew the chart's border, has also
  been removed. It was only being used for debugging but it might be a
  good idea to add this an optional feature...
Fix bug in PieChartColor::toHex()

This bug would return the CSS colour without any leading zeros, e.g.
0xff => '#ff'; which would cause problems for any colours with a
hexadecimal value with less than 6 digits.
Refactor PieChartImagick to extend PieChart

PieChartImagick has been made to extend the PieChart abstract class and
use the new PieChartColor class. The methods that PieChart has made
generic are no longer needed and have been removed. The duplicate
comments have also been deleted.
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.
Update README.md

The changes to the structure of the project meant that README.md had to
be changed slightly, so as to still be relevant; 'index.php' was renamed
to 'example.php' to prevent it from being used unintentionally.