Added metadata field

First off: thank you for this library.

According to the [Cisco Specifications](http://www.cisco.com/en/US/technologies/tk648/tk362/technologies_white_paper09186a00800a3db9_ps6601_Products_White_Paper.html), the different flows are identified by both the source_id and the source's IP Address, because the flow_id can be repeated across multiple devices.

So I added to the packets returned by the library a "metadata" field, which includes: 
* address: "192.168.1.1"
* family: "IPv4"
* port: 64125
* size: 190

This way we can use address to identify different flows across devices.