Add support for MySQL TIMESTAMP field
You can always implement a custom field type: http://peewee.readthedocs.org/en/latest/peewee/models.html#creating-a-custom-field

I do not have any plans at the moment to implement the timestamp field, however.
 For future reference, MySQL's `TIMESTAMP` field behaves almost like `DateTimeField`, so a trivial implementation looks like this:

```
# Custom field, behaves just like DateTimeField.
class TimestampField(DateTimeField):
    db_field = 'timestamp'

MySQLDatabase.register_fields({'timestamp': 'TIMESTAMP'})
```

However, this implementation is likely incomplete. In special, a `DATETIME` field stores the date in the exact format passed to the database, while a `TIMESTAMP` field will convert to UTC while writing, and convert from UTC to local time in a SELECT result.

For a simple order_by, I can use `TIMESTAMP` and `DATETIME` the same way. For anything else, we might need extra code.

Peewee already has support for `datetime` field, but if I'm working with a "legacy" database, it would be desirable to also have [`timestamp` field](https://dev.mysql.com/doc/refman/5.5/en/datetime.html).
