Support jumping around a gb-based project.

[abourget]

See constabulary/gb#53 and constabulary/gb#93 and constabulary/gb#42

Maybe it could automatically add to the $GOPATH the directories with vendor/ all the way up the dir tree ? Would that be acceptable ?

[thomasf]

Also maybe relevant for the go1.5 experimental vendoring support.

[fatih]

I've implemented GB support inside vim-go which modifies the GOPATH automatically if it detects that you are working inside a GB project. And it works perfect with godef. Just wanted to add it as a side note

[rogpeppe]

I think that fatih's solution is probably the right one. I don't think godef needs to know about gb.