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.
Merge pull request #3 from sealevelresearch/handle-inconsistent-results

Add `download_recent_measurements` function