Limit lone pair drawing to Nitrogen atoms only.

There are still some issues with positioning of the lone pairs.  For instance
1 N 1 1 {2,S} {3,S}
2 H 0 0 {1,S}
3 H 0 0 {1,S}

Will draw the long pair on the hydrogen rather than the N.  But for now, we can
avoid drawing lone pairs on oxygens and such where it is not needed.