where() calls are not chainable with update()
Thank you so much for bringing this to my attention, you are absolutely right.

The documentation (http://peewee.readthedocs.org/en/latest/peewee/api.html#Query.where) states that "where() calls are chainable. Multiple calls will be “AND”-ed together.".  This works for SelectQuery:

```
Model.select().where(Model.id == 1).where(Model.status == 5)
SELECT ... FROM `model` AS t1 WHERE ((t1.`id`=1) AND (t1.`status`=5))
```

But does not work for UpdateQuery - only the final where() clause is included in the generated query.

```
Model.update(status=-1).where(Model.id == 1).where(Model.status == 5).execute()
UPDATE `model` SET `status`=-1 WHERE (`status`=5)
```

In this example the Model.id == 1 clause has been dropped, causing a dangerous update.

The clauses are all correctly included if they are specified in a single where():

```
Model.update(status=-1).where((Model.id == 1) & (Model.status == 5)).execute()
UPDATE `model` SET `status`=-1 WHERE ((`id`=1) AND (`status`=5))
```
