Substructure highlighting in pandas dataframe (fixes #362)

Added highlighting of matched substructures in the pandas dataframe
rendering in PandasTools (Issue #362 )

NOTE: Pull request #401 (Fixed LoadSDF to keep 3D info, @samoturk) was kept independent from this pull request. I would suggest to merge it into my fork and combine it with the SDF export addition and create a separate pull request combining the SDF I/O changes.