Adding owner and group attributes

# Problem

By default, the $GOPATH directory is created as root. This means that `go get github.com/xxx/xxx` commands need to be run with sudo in order to install packages.

# Solution

I've added `node['go']['owner']` and `node['go']['group']` attributes that will be used for creating $GOPATH and when calling the `golang_package` provider.

In order to not break any existing implementation, I've defaulted both of these attributes to _root_. The README has been updated to show this.