Update bn.cfg

[MarnixCroes]

quite a few changes and new additions, so it's worth updating it

[coveralls]

coverage: 93.229%. remained the same
when pulling 42f0a32 on MarnixCroes:example-config
into d17d67b on libbitcoin:master.

[evoskuil]

The probably isn't the best way to be providing a sample config, since every setting will have to be changed just for it to load. I would recommend using this as a basis for providing markdown documentation and instead for the sample, provide a minimal useful config file.