Add support for a variable number of callback arguments

[CameronDevine]

This pull request allows callbacks to be written with between 0 and 4 arguments. Python's inspect.signature() method is then used to determine the correct number of arguments to provide to the function. In order, the provided arguments are,

1. the message,
2. the topic name,
3. the headers (which include a timestamp), and
4. the client itself.

[rhytnen]

I think this approach is overly complicated and it leads to 2 issues.

1. Keywords are now order dependent (so why are they keywords?)
2. Variable names have to be precise

I think if you want to keep this approach, you would be better off iteratively mapping values to parameter names instead of a comprehension and range. You could then even get a little bit extra and check if any argument name is a key to the message and extract it or something like this.

However, my approach would be to keep at most 2 signatures.

• f(topic, message)
• f(topic, message, timestamp, io)

Since the header is in the message, I wouldn't extract it.

[CameronDevine]

I decided not to include a reference to the Pigeon class in the callback arguments. If this is something you would like to do, it can be added at the time of subscription using functools.partial. A bug which caused an exception when partial was used has also been fixed here.