Gracefully handle no sleep data having been recorded for the specified date

Missing sleep data for a particular date would cause the following error to be raised:

```
fitbit-to-graphite.rb:153:in `extract_data': undefined method `[]' for nil:NilClass (NoMethodError)
	...
```

This change attempts to identify that situation and output a helpful message instead:

```
No sleep data recorded for 2014-04-14
```