Add Check for Zlib in PHP Config Check

Hello,

I was able to get piwik up and running. Thanks for the software and help!

I do have one suggestion for improvement. After I started getting data, the graphs wouldn’t display right. It turned out I didn’t have the ZLIB module included in PHP. Once I included that everything worked.

A check for the ZLIB library when the install wizard checks for other PHP libs would prevent other people from experiencing this issue.

Mark