Setting up mask outside data range raises an error

Not really a bug, but an annoyance.

When specifying a mask with 
```
raw.add_mask_period(start, end)
```
If start or end falls outside the data range, an exception is raised:
```
ValueError: Attempting to set the stop time of a mask period after the actual stop time of the data.
```
Forcing to add a code to crop periods to the data.
This code could be executed by pyActigraphy instead.
