More cleanup related to issue https://github.com/fatih/vim-go/issues/222

 - extracted the code to detect the "default" GOPATH into it's own function;
 - expand() automatically takes care of trailing slash if given, this allowed to remove the explicit check for trailing slash (which remained non cross platform friendly);
 - this allowed GoBinPath() to be nicely cleaned up. Plus the new functions may be used in other places.