Add NAT support

[thanhvg]

This PR adds NAT/proxy support.

Currently after _getCapabilities() succeeds node-onvif will resolve to use direct ip address of the devices and ignore the xaddr/addess passed in by the user in constructor OnvifDevice

This is undesired if the device is behind a NAT or is routed by an nginx server with multiple subpaths such as 10.10.1.2:8000 or 10.10.1.2/route-to-device/. node-onvif won't be able to connect.

Change in this PR allows user to pass in ip address or url pathname as the preferred connection string so node-onvif will always use this value to connect to the device and ignore the device's internal ip address.

For example:
OnvifDevice({ address: '10.10.2:300', user: 'user', 'pass': 'pass'})
OnvifDevice({ address: '10.10.1.2/route-to-device', user: 'user', 'pass': 'pass'})

Tested with node 6 and 8

[futomi]

Sorry my action is late. I'm grateful for your PR.