Cascade delete is not working as expected
Looks like sqlite by default has foreign key support turned off:

http://stackoverflow.com/a/13641365/724251

Need to add something like:

```python
Tweet.raw("PRAGMA foreign_keys=ON").execute()
```

Might be a nicer way included somewhere in the peewee docs. @alexlatchford 
thanks for pointing it out. 
Seems to be mentioned in the documentation, in http://docs.peewee-orm.com/en/latest/peewee/database.html?#additional-connection-initialization

Although imho it is a little bit scary, that referential integrity is turned off by default.
 @aellwein -- don't use SQLite if you don't know how it works. Simple as that.
Hi Charles,

i created a table which is related to another table by a ForeignKeyField and on='CASCADE' policy in Sqlite.
It seems like if i delete a record from the parent table, the children are not deleted.
Here is a test that proves that behavior:
https://gist.github.com/aellwein/9e814769196e5524e94199163aa4a0bd

Am i doing something wrong?
Thanks!
Alex
