foundations for rich display (prototype DataDisplay module, to be merged into Base)
handle multi-line cells as a block
treat empty cells as silent
some more eval_cell fixes
implement IPython-like In/Out history
added display_text (text/plain) to display machinery, some rearrangements; only display 1 arg at a time (at least for now), as allowing display(x...) led to stack overflow in some cases because Julia gets confused about which routine to call