Merge pull request #3205 from minrk/traitletsargs

add ignored *args to HasTraits constructor
Remove references to HTMLCell (dead code).
Merge pull request #3221 from dwyde/remove-html-cell

Remove references to HTMLCell (dead code)

Closes #2980.
Merge pull request #3089 from minrk/stdin

add stdin to the notebook
allow formatters to publish metadata

formatters can now return either data or (data, metadata)

the resulting published dict has both content.data keyed by mime-types,a
and content.metadata keyed by mime-types.

metadata keys are not necessarily defined, and are generally undefined when they would have no content.

This fits fully within the existing message spec.
publish image shape as metadata
support display_pub metadata in js frontend
format-related docstrings!