adding protocol attribute, closes #2

WG thread:
http://lists.w3.org/Archives/Public/public-web-perf/2014Oct/0059.html
protocol -> networkProtocol

Renaming based on feedback from:
http://lists.w3.org/Archives/Public/public-web-perf/2014Oct/0093.html
rename and initiliaze

- networkProtocol > nextHopProtocol (based on feedback @ TPAC)
- initialize value to empty string
Merge pull request #4 from w3c/protocol

Adding nextHopProtocol attribute