Add writeData and readData to NS(In|Out)putStreams.

writeData:(NSData *) lets us write NSData objects from Nu, while
(NSData *)readData lets us read NSData objects from Nu. This lets us use 
input and output streams from Nu, since otherwise they deal in raw byte 
arrays that we can't manipulate easily from there.

These are put in Nu categories; let me know if you'd like them moved elsewhere.
I'm using them to write bindings to ensime for background Scala compile
verification.