NEW ISSUE: default binding for callback

[Anish Karmarkar <Anish.Karmarkar@oracle.com>]

Title: default binding for callbacks

Target: SCA assembly specification

Description:
In resolving issue 1A, we reaffirmed that when a binding is not 
specified for a service it defaults to binding.sca. OTOH for references 
we decided that there won't be a default binding. Instead, any of the 
service bindings to which such a reference is wired to, can be used.

Bindings can also be specified for service/callback and 
reference/callback elements. We don't say whether there is a default for 
such bindings when the callback element is not specified.

Proposal:
I believe we don't need a default for callbacks. When a binding is not 
specified it is unconstrained (any binding as opposed to binding.sca). 
We should add a statement regarding this in the spec.