Derive "diesel::Identifiable" if struct has a primary-key

[hasezoey]

This PR adds derive diesel::Identifiable to all Read-structs which have a primary key.

fixes #133

Note: personally i thought the Identifiable is only necessary for Associations or if you want to use the struct directly in a update (see doc quote)

This must be implemented to use associations.
Additionally, implementing this trait allows you to pass your struct to update (update(&your_struct) is equivalent to update(YourStruct::table().find(&your_struct.primary_key())

source

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@longsleep does this properly fix you case? also could you explain why it would be necessary aside from Associations or is the second "helper"(the update(&struct)) the reason?

[longsleep]

@longsleep does this properly fix you case? also could you explain why it would be necessary aside from Associations or is the second "helper"(the update(&struct)) the reason?

Users of the generated structs might need the Identifiable trait in their own code (like to get access to the id function or for other purposes).

[longsleep]

Would be great to finally get this as part of a release.