events(bitswap) try the new event logger in the bitswap GetBlock method

@jbenet
@whyrusleeping

Let me know if you want to direct the eventlog output to _both_ the file
and stderr. Right now it goes to file. Perhaps this is just a minor bip
in the larger discussion around log levels.

https://github.com/jbenet/go-ipfs/issues/292

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>