Update PieChartImagick.php

Added "use" to class Imagick and ImagickDraw.

"Error: Class 'SamChristy\PieChart\Imagick'" in Symfony project.
Merge pull request #2 from tiagokalinke/patch-1

Unqualified function names are resolved globally but unqualified classes are *not* - which is why PieChartGD.php worked but PieChartImagick.php didn't (http://www.php.net/manual/en/language.namespaces.rules.php) - another reason to love PHP!

Thanks for the patch, mate. :)