Add status query methods

Adds methods for querying the status of bill, subscription, and pre-authorization objects.

Before:
```ruby
if subscription.status == 'cancelled'
```

After:
```ruby
if subscription.cancelled?
```

Apart from being easier to read/write it also eliminates a potential source of error, i.e. in the first example a misspelt status value would fail silently, whereas in the second example a misspelt method name would raise an exception.