Enable read/write tables in raw mode when not managed

[bsweeney]

The default table handling logic for non-managed (within php-font-lib) tables should be to read/write the table in raw mode. Table::parse and Table::encode have logic to read/write the table in "raw" mode if the table structure ($def) is not defined. However, that logic is currently disabled because not all managed tables have a $def.

Originally posted by @bsweeney in #115 (comment)