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

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
```
