Fix up regression in promoting lookups to extensions

Was broken by 6ed2eb409231a7fe3e0c2a135f9a89796206f639

Fixes https://github.com/behdad/fonttools/issues/110
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
Minor
[merge] Improve assert
[merge] Rename
Make tag parameter to table constructor optional

If not provided, extract from class name.
[merge] Support missing GSUB/GPOS/etc

This is a hack for now.  Will clean up in upcoming commits.
Minor
[merge] Rename
[merge] Install merged dict on object
[merge] Allow merging OS/2 tables of different versions
[merge] Comment
[merge] Minor
[merge] Minor
[merge] Minor