Better `saved_file` documentation

[SmileyChris]

When saved_file signal actually fires was the confusion in #185.

Docs should clearly show that this signal only fires when an uncommitted file is attached to the model and then the model is saved.

[orientalperil]

How do you attach an uncommitted file to a model?

This sample code is what I'm currently using. I am sure this is an incorrect usage of the API. I had to manually set the _committed flag to false and hardcode the path 'temp/Earth_from_Space.gif' to make the saved_file signal get sent. How would you write this code correctly?

# Profile.photo is a ``ThumbnailerImageField``
from easy_thumbnails.files import ThumbnailerImageFieldFile

profile = Profile.objects.get(pk=1)
file_field = dict([(x.name, x) for x in profile._meta.fields])['photo']
t = ThumbnailerImageFieldFile(profile, file_field, 'temp/Earth_from_Space.gif')
t._committed = False
profile.photo = t
profile.save()