Protein structure

Hi,

Just wanted to inform everyone on the secondary structure rendering
progress. The Protein class can now extract the HELIX records from a
pdb file (actually, from the Molecule’s properties). If these are not
available, we can still try to find it ourselves.

I improved the rendering of aplha helixes (see screenshot) but it
should also be possible to draw a nurbs through the points? I’ll
probably split the current ribbon engine up into a ribbon and cartoon
engine tonight.

http://imagebin.org/39109

Tim