Add POM file

Was missing from git, woops!
Add remaining socket factories.

Adds support for:

  * DEALER
  * ROUTER
  * PAIR
  * PUSH / PULL
  * PUB / SUB
  * REQ / REP

Note: `XSUB` and `XPUB` are missing, as it appears these have been
deprecated/removed from the upstream API.