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.
Draw lone electron pairs only for nitrogen containing species
Make transport writing compatible with Python 2.6

The format syntax where you omit the field numbers 
was introduced in Python 2.7.
Merge greengroup/master

nch 'greengroup/master' into scoop

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