Add public method to access _processors in BoundLogger

[heikoheiko]

My use case here is, that I have one special processor wich forwards events to listeners, independent of the loglevel. in order to check if the logger 'is active' i need to access the first processor in list and check if it has listeners. currently i have to access the private member _processors.

i think a pythonic way would to solve this would be to simply make the processor list public.

[heikoheiko]

related to ethereum/pyethereum#202

[hynek]

Ok, I will document it as public similarly to _logger. I will not change the APIs though because turns out people are using internal APIs. ;)