Close #206: greytrack_font_rotation is linear only

See Issue #206

I confirmed that greytrack_font_rotation is only used by the Linear diagram and added comments to that effect in the code. 

However, I don't see the source for the old documentation [1] offhand, so that is not specific.

1: http://biopython.org/DIST/docs/GenomeDiagram/userguide.pdf

P.S. At an @openhatch event! (task 71)