Resolve relative URLs in GM_openInTab.
Resolve relative URLs in GM_openInTab.

Fixes #1719

Martii/greasemonkey,Martii/greasemonkey,HasClass0/greasemonkey,the8472/greasemonkey,Ventero/greasemonkey,Ventero/greasemonkey,HasClass0/greasemonkey,the8472/greasemonkey