Unexpected loading and attaching sna

[mbojan]

fortify.network unexpectedly loads and attaches the "sna" package even though "sna" is declared only as import. That might ruin your workspace because of several name conflicts.

IMHO load_pkg is a rather nasty hack with even nastier side effects ;) I think it is only needed once to call an appropriate layout algorithm from "sna".

I will submit a PR.

[briatte]

Fixed by #10.