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@mappum, could you CR this and let me know if I've interpreted the
desired behavior correctly?

NB: return variables are provided for clarity. Otherwise, it's not
entirely clear that the bool is for _found_.

TODO add tests for remaning, untested methods.

@mappum this optionvalue is a really great abstraction. It makes
maintenance super simple!

@jbenet @whyrusleeping

we've gotta allow the code to speak for itself. I wouldn't have been
able to safely modify this code without having my hand held. I am but a
lowly programmer with a simple mind.

cc @jbenet @whyrusleeping @mappum

commands: Made Command run functions return (interface{}, error) instead of setting the values in the response

commands/http: Don't set Content-Type for stream outputs so browsers can MIME-sniff the actual content type