Webapp: Add `since=<timestamp>` query param handling on GET calls

[rhyolight]

Now that I've got a lot of data in the HTM Engine, I don't want to get all of it on every GET.

Example:

curl http://localhost:8080/177

Gives me all data for model 117, but I only want it since Sat Jun 27 2015 (timestamp 1435388400):

curl http://localhost:8080/177?since=1435388400