text: Title: Peewee backend doesn't commit by default
  Body: I found this surprising, and it seems like a bug, but the Peewee backend doesn't save by default. It seems like it should default to committing, like the Django backend does. Here's a simple test case:
  
  ``` python
  from mixer.backend.peewee import mixer
  from models import Foo
  print(Foo.select().count())  # 0
  mixer.blend(Foo)
  print(Foo.select().count())  # Still 0, which was unexpected.
  ```
  
  Doing this works just fine, but it's not documented anywhere that this is required:
  
  ``` python
  with mixer.ctx(commit=True):
      foo = mixer.blend(Foo)
  ```
  