Add debug logs to take-snapshot.js

I've been trying to debug why (in some cases), images aren't fully
loaded. @lencioni saw this while snapshotting his twitter profile,
https://twitter.com/lencioni, and I managed to reproduce. To better
understand what's going on, I've added some debug logging that you can
turn on when manually taking a snapshot.

By default, the logs don't show up. But if you pass in `debug: true` in
the json argument, they are logged to standard out. Here's an example:

phantomjs ./script/take-snapshot.js
"{\"address\":\"https://twitter.com/lencioni\",
\"viewportSize\":{\"width\":1000, \"height\":1000},
\"outfile\":\"/tmp/screenshot.png\", \"debug\": true}"

After doing this, I spent a lot of time trying to reproduce the problem
from the command line (with the above command). But I was never able to
do get the same issue to appear. I'm adding these debug logs
anyway, to allow others to try it out and see if they can find out
what's going on.