Add `download_recent_measurements` function

It turns out the EA website gives you different results if you ask multiple
times (see analysis here 
https://gist.github.com/paulfurley/693e9d466947cc979468)

This new helper function asks for the page 10 times and yields unique 
results. That means you can do something like

```python
for result in download_recent_measurements(station_id):
   push_to_database(result)
```

... and you'll get the results as quickly as possible, with potentially 
multiple results if the page returns different values each time.