Let Timestamps plugin use Tumblr API v2

I tried to rewrite the Timestamps plugin (or at least the part that was throwing insecure content errors) to use Tumblr API v2 over HTTPS, in response to [recent issues with Chrome beta](http://xkit-extension.tumblr.com/post/96164537552/xkit-chrome-betas)

However, this does involve some collateral damage: unfortunately, the v2 API [requires an API key](https://www.tumblr.com/docs/en/api/v2#posts); here I used the sample key found in the documentation. Obviously XKit should have its own key - but I'm not sure how problematic that would be. (And it's probably best to find somewhere else in the code to put the API key.)

Also, are you still updating the Github repository? I notice it's quite a bit behind. I'd be happy to help rewrite other things to use the v2 API if necessary...