move raw cell inclusion to null.tpl

select based on raw_format metadata key if defined.
Default behavior is to include raw cells of unspecified format (consistent with existing behavior).
add raw_format to Exporter classes

sets their values in resources
test raw cell inclusion based on raw_format metadata
rename Raw Text to Raw NBConvert in menu
tweak global hide/show

make hierarchical rather than either/or.  Also, don't use the same class name for the outer and inner css classes.
give Raw Cells a placeholder

and make sure cell_type is defined *before* calling TextCell.apply
clarify name of 'default' cell toolbar
allow cell toolbar presets to apply only to specific cell types
add raw cell toolbar preset
add dialog to raw cell toolbar
set cell_type properly first-class in the Cell object

- in toJSON
- don't clobber existing value in Cell constructor
fix Python mime-type
propagate raw_mimetype to nbconvert
tweak raw placeholder
use reST in raw cell toolbar

reduces width
Merge pull request #4306 from minrk/raw-cell-metadata

add raw_mimetype metadata to raw cells