added device= method to set the device based on name or number

For accounts with multiple devices, this allows a device to be specified by either name or number (0, 1, 2, etc):

```ruby
@nest.device="Main"
@nest.temperature # => 68.0

@nest.device=1
@nest.temperature # => 65.0
```

I've tested this and it works great. Unfortunately I don't have specs for it.