Made write truly asynchronous, bypasses bug in pyserial which made it…

[wolfmanjm]

… blocking.

Also added a flush command which will clear the write buffer.

Addresses issue #53

[ulysse314]

Thanks for this patch, this was useful for my project. I would be happy to see this patch merged into pyserial-asyncio.

[Nable80]

It took some time to understand why 2 almost identical branches were used:

        if self.get_write_buffer_size() == 0:
            self._write_buffer.append(data)
            self._ensure_writer()
        else:
            self._write_buffer.append(data)

I think it's natural to simplify (and clarify) this block to be something like this:

        prev_buffer_size = self.get_write_buffer_size()
        self._write_buffer.append(data)
        if prev_buffer_size == 0:
            self._ensure_writer()

Do you agree?