Support for NewType/transparent structs

[jakajancar]

Any chance of this from the old repo being merged?

sfackler/rust-postgres-derive#18

[malobre]

I rebased the linked PR and did some minor modifications on my fork: malobre@8270a78. So far it's working great.

@sfackler If can submit a PR if you want.

[sfackler]

Sure, I'd be happy to take that feature!

[malobre]

I submitted #857, however there are still a few changes I'd like to make.