Specify encoding for reading notebook file.

Closes gh-4562
Specify encoding in remainining instances of io.open
Merge pull request #4563 from takluyver/i4562

Specify encoding for reading notebook file. and a few other calls to io.open with missing encoding
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