add icon images for ontology classes.
move vector graphics into assets directory.
Merge remote-tracking branch 'origin/staging' into augmenting213_ontology_class_icons
include ontology-class icons into fancy links.
center the icon for single ontologies.
use ontology-class icons for ontology title line.
adjust sizes of ontology-class icons

- also remove inline-css for the ontology
  title h2 and use a css-id instead.
Merge pull request #266 from ontohub/augmenting213_ontology_class_icons

Ontology class icons; fixes #213.