Added Gavin Llewellyn to THANKS.md file
Added Gavin Llewellyn to THANKS.md file

- Patch applied from master

joseph-onsip/SIP.js,josephfrazier/SIP.js,Escaux/SIP.js,markandrus/SIP.js,onsip/SIP.js,ronlobo/SIP.js,Voiceworks/SIP.js,onsip/SIP.js