add bufferStream option again, add streaming autoOrient test

for autoOrient to work, we're going to need to somehow buffer the stream.
since 99% of the time you don't want to buffer the stream, buffering it should be opt-in, thus it shouldn't be removed.

at least this time, it's backwards compatible, so gm v2 is unnecessary. unfortunately, it drops support for node < 0.8 because we use `Buffer.concat()` (though that may be a reason to bump to v2).

in short, we basically just made the bufferStream option better.