This adds a valid_until method to the totp object.

This enables one to get the time that the code returned from totp.at() will be
next generated (assuming that you pass the same time object to both methods)

```python
In [1]: totp = pyotp.TOTP('wrn3pqx5uqxqvnqr')

In [2]: date = datetime.datetime(2014, 7, 21, 14, 49, 3)

In [3]: totp.valid_until(date)
Out [3]: datetime.datetime(2014, 7, 21, 14, 49, 30)
```