*** primus: Add new binary "primus-nvidia" package.

Rationale is as follows:
1) The current primusrun script provided by upstream PPA assumes that user
   wants to use primus with proprietary NVIDIA driver. It _is_ possible to
   use primus with nouveau, but the current packaging does not support this.
2) Debian Policy 2.2 (http://www.debian.org/doc/debian-policy/ch-archive.html)
   forces us to support both nouveau and nvidia, unless we want all of primus
   to get dumped in contrib + non-free (= Ubuntu multiverse). Besides, we
   want to make it possible for users to choose both free and non-free
   alternatives.

Implementation:
1) New empty binary package "primus-nvidia", à la bumblebee + bumblebee-nvidia.
2) 2 primusrun scripts provided; users switch between them with
   update-alternatives mechanism, or by choosing whether or not to install
   primus-nvidia.
3) We assume that users who install primus-nvidia explicitly (because nothing
   pulls it in automatically) want to use the proprietary driver, so
   primusrun-nvidia script has higher priority than primusrun-nouveau.

Sorry for this large commit, but it's not quite possible to cherrypick stuff
out from this commit anyways.