Add isUnicode() and isSymbol() to cmap subtables
[merge] Use CmapSubtable.isUnicode()
[subset] Use CmapSubtable.isUnicode()/isSymbol()

Fixes https://github.com/behdad/fonttools/issues/111
Add NameRecord.isUnicode()
[subset] Use NameRecord.isUnicode()
[subset] Correctly handle subsetting non-Unicode cmap subtables