Make add and remove callbacks take lists of message for convenience

[mfine]

Check to see if the msg_type is a list, and then apply the callbacks to all the message types. Better way to do something like this?

/cc @fnoble @denniszollo

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[fnoble]

This could be generalized to all iterables. The most "pythonic" way to check if something is an iterable is just to assume it is and catch failure:

try:
  for mt in msg_type:
    self.callbacks[mt].add(callback)
except TypeError:
  self.callbacks[msg_type].add(callback)

http://stackoverflow.com/questions/1952464/in-python-how-do-i-determine-if-an-object-is-iterable

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[mfine]

Moved to iterable and added some tests.

[fnoble]

Still a "List" here

[fnoble]

But maybe that more instructive?

[denniszollo]

Love it

[fnoble]

lgtm

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[mfine]

Fixed the docs. Bringing in.

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