Implement DQT merging

See #30.

It seems to work correctly, reviews welcome. Saved me a whopping *four bytes*!
The intended return value should be the same as the result of calling emit_dqt() in a loop (https://github.com/dwbuiten/mozjpeg/blob/dqtmerge/jcmarker.c#L575) -- that is, +1 for every non-8bit table.