Go not setting $GOPATH by default

[swedebugia]

Hi

I have just installed go and it seems that it does not set a gopath by 
default.

This leads to errors when trying to use go get, e.g.:

address@hidden ~$ go get github.com/syncthing/syncthing
package github.com/syncthing/syncthing: cannot download, $GOPATH not 
set. For more details see: go help gopath

I propose that we set the gopath by default to "~/go" and create that 
directory by default, what do you think?

cheers
sdb