Move port mapper out of core and into network drivers




```bash
=== RUN TestSetIptablesChain
--- PASS: TestSetIptablesChain (0.00 seconds)
=== RUN TestMapPorts
--- PASS: TestMapPorts (0.00 seconds)
=== RUN TestGetUDPKey
--- PASS: TestGetUDPKey (0.00 seconds)
=== RUN TestGetTCPKey
--- PASS: TestGetTCPKey (0.00 seconds)
=== RUN TestGetUDPIPAndPort
--- PASS: TestGetUDPIPAndPort (0.00 seconds)
PASS
coverage: 80.4% of statements
ok      github.com/dotcloud/docker/networkdriver/portmapper     0.005s
```