Rename OutputSource to something more semantically correct

[sbooth]

OutputSource is not a great name for the class. It does not represent a source of output but rather is a destination where output is written.

For parity in naming between input and output here are some ideas:

Input	Output
SFBInput	SFBOutput
SFBInputReader	SFBOutputWriter
SFBInputProducer	SFBOutputConsumer
SFBInputSource	SFBOutputDestination