Compatibility with sdf files served by the PDB

SDFs provided by the PDB (Protein Data Bank) have a slightly different format than what RDKit is expecting.

http://www.rcsb.org/pdb/download/downloadLigandFiles.do?ligandIdList=XK2&structIdList=1HVR&instanceType=all&excludeUnobserved=false&includeHydrogens=false