export charts as image

[mwhudson]

It would be super nice to be able to export charts as images. jqplot appears to support this, with the jqplotToImageStr function, but that doesn't seem to be in the version that's bundled with codespeed.

[tobami]

ah! i didn't know that. Seems like a motivation to update the jqplot version...

[str4d]

Since #199 was merged, we now have access to this function!