Fixing Key Signature Meta Event

In the "key signature" meta event, the key is defined by a signed 8-bit integer between -7 and 7 (-x = # of flats and +x = number of sharps).

Stream.js was only reading unsigned integers so a key signature of F (1 flat) was being read as 255 instead of -1. I added an optional parameter to readInt8 to convert the int to a signed int. 

I didn't check if this param would be useful in other cases. I'm under the impression that none of the other files use this event in any way so it shouldn't mess anything else--just correctly set the even value.