Fix retrieving model for reverse relations

[glowka]

Hey,

current util get_related_model implementation does not take into account that reverse relations (descriptor can be both forward and reverse) so for reverse relations it leads to bugs like this:

class Member(models.Model):
    pass

class Project(models.Model):
   members = models.ManyToMany(Member, related_name='projects')

get_related_model(Member, 'projects')
# returns models.Member instead of models.Project

New implementation robustly checks type of relation and returns appropriate model.

It supports all kind of descriptors: OneToOne, ManyToOne, ManyToMany.