Add SSH port setting

You can now set SSH port when indexing documents over SSH:

```sh
curl -XPUT 'localhost:9200/_river/mysshriver/_meta' -d '{
  "type": "fs",
  "fs": {
	"url": "/tmp3",
	"server": "mynode.mydomain.com",
	"port": 22,
	"username": "username",
	"password": "password",
	"protocol": "ssh",
	"update_rate": 3600000,
	"includes": [ "*.doc" , "*.xls", "*.pdf" ]
  }
}'
```

Closes #50.