Issue 19 fix: Pass #2: Pass positional arguments down to stdlib

Ref: https://github.com/hynek/structlog/pull/23

Apologies for opening another PR for same issue. I didn't want to pollute the original author's PR. 

I addressed all of your comments that I agreed with and added a commit. I retained the original author's commit as well. Tox tests pass on my box. 

I disagreed with one comment about adding pass through methods in stdlib BoundLogger (https://github.com/hynek/structlog/pull/23/files#diff-519ce9a9803f8404219d01de0afb0c1fR65). There are more than 5 such methods that are in stdlib logger. I was reading the code for the first time and instantly got what getattr was trying to do. I don't feel like it's magic. If you still insist on explicit pass through methods, I can add a commit. Please let me know. 
`del event_dict["positional_args"]` would be more idiomatic here I think.