Add static image charts

Google Charts must be generated client side, which stops them from being embedded in e-maiils, pdfs, etc.

This pull request uses phantomjs and imagemagick to render the charts, take a screenshot, and return a PNG image on the server.


Done:
*  imagechart report format `/report/imagechart/?report=...`

TODO:
*  detect if phantomjs is installed on the system and throw an error if it's not
*  attach/embed chart images in email
*  create PDF format with embedded chart images
*  add download as option for image charts
*  update documentation