Add code for logging image loading times

Change-Id: Ibeb5d1cd6cd062afef9b547b680e43540bd7c312
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/816
Track image loading time via onload

Adds support for browsers which do not have the Resource Timing API.
In my machine, onload-based loading times are inaccurate to the point
of uselessness, but that might be related to HTML page generation
being extremely slow; maybe with different conditions it will prove
more useful.

Change-Id: I8516f409451a224e5d16ce3b50eb4f720ffc00ad
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/816