IRuby Notebook integration

[zverok]

There should be isolated (not required by default) lib/reality/iruby module, which, on require adds #to_html method to Entity and List, making them pretty for IRuby Notebook. And maybe Entity#describe should return some object, which is inspect-ed as a multiline string (for irb) and to_html-ed as, well, pretty HTML?..

[zverok]

...and also List#to_dataframe(list, of, properties) seem to be a good idea.