ManyToManyField in Shortcuts
Yeah, it's currently only in master. I will be pushing a new version soon which will contain the field.

ManyToManyField in Shortcuts module is not working in 2.4.5

``` py
from playhouse.shortcuts import ManyToManyField
```

yields in:

```
from playhouse.shortcuts import ManyToManyField
ImportError: cannot import name 'ManyToManyField'
```

So it finds the module, but not the class ManyToManyField...

Note: I refer to the example at:
http://peewee.readthedocs.org/en/latest/peewee/querying.html#manytomanyfield
Where the import statement is used.
