added test for display() calls with metadata

this test is currently failing
propagate display metadata to all mimetypes

This change makes:

    display(SVG(s1), metadata=dict(isolated=True))

behave the same as

    display_svg(SVG(s1), metadata=dict(isolated=True))
back out metadata copying for every mime-type
make output code not drop non-mimetype-keyed json
use non-mimetype-keyed metadata in the outputarea
clear output in-between test runs
passthrough unrecognized output mime-types

preserves the bad mime-type -> short name conversion for supported types,
but allows passthrough for additional mime-types,
so at least data is not discarded.
check mime-keyed metadata first, then top level
distinguish display_svg() and display() tests
use mime type in memory
align with current master
convert short names like 'png' to mimetype names
fix png inlining
stop using convert_to_mime_types

the in-memory representation and the messsage spec now all use mimetype
names for the JSON keys.
convert back to short names when writing to JSON
unify key renaming for to/from json short keys
minimal notebook rountrip test
add (failling) roundtrip test case
fix toJSON/fromJSON renaming
minor formatting