Support bytes as return type

[anandology]

Add support for functions returning bytes.

It should be possible to run the following function using firefly.

def random_bytes():
    return b'hello'

It should set the appropriate Content-Type. I think the suitable content-type is application/octet-stream, but it is already used for sending files. I think the file type should pick something other content-type.

[droidlife]

I think the file type should depend on the extension of the file. If the file extension doesn't exist then we might use application/octet-stream.