New improvements to affs-authors issue.

As it was impossible to use model_formsets for adding authors as it was
complicated to filter out only those affiliations that belonged to one
particular abstract I have completely split Authors and Affiliations
editing. I have overriden the __init__ method on AuthorForm to filter
only Affiliations for the current abstract. Also the save method of the
Affiliation model form to always connect the affiliation to current
users abstract.