Fix signature after and before create methods

[golenkovm]

This is to fix the issue #30

[golenkovm]

It seems like signature class does some tricky things. Having existing default signature when a new default signature is being added it adds it first, then in get_default_signature_for_user() it gets both signatures, but returns only one. The order it gets signatures is not specified so due to DB internal reason it may be unexpected order. Sometimes it gets newer record and makes it not default, but then in after_update() it makes it default again. Tricky logic. I tried to simplify this and it seems like can just fit everything into before_create().
Looks like it now works as expected.