Save document as a sequence of bytes

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
Use only one digit for miliseconds instead of 3

It is not really necessary higher precision.
Revert to default Python (2.x) and import functions from future

Not all modules that transcribe depends on are ready for Python3 yet.
Fix spaces and typos
Remove trailing space and carriage return when inserting time marks
Add accelerators for F keys