Make U+00A0 act as space in math mode.  Resolves issue #377.
Make mfenced match mrow+mo form, and make U+2223 and U+2225 stretchy in pre- and postfix positions (to match Firefox).  Resolves issue #478.
Merge branch 'issue377' into develop.  Issue #377.
Merge branch 'issue478' into develop.  Issue #478.