Concatenate NSInputStreams

Some really useful code from guys at couchebase labs, allows to concatenate any number of input streams into a single input stream

https://github.com/couchbaselabs/TouchDB-iOS/blob/master/Source/TDMultiStreamWriter.h

Subclassing NSInputStream writer is not as easy as you would think, especially since bunch of this stuff is not documented anywhere that I could find.