[WIP] Add support for PHP Metrics #521

Hi,

This PR adds support of [PhpMetrics](http://www.phpmetrics.org)

+ bubbles chart
+ metrics trends (maintenability, LCOM, number of bugs...)

**I need some help to finalize this PR**. I would like to integrate some image/html file in the build (as does [PDepend](https://github.com/Block8/PHPCI/blob/master/PHPCI/Plugin/Pdepend.php#L108) ), but these assets are not accessible from public. Is there a tool to expose assets in build ?