Set pipe size issue322

[SilverPlate3]

Possible Solution to #322.
Including unit tests.

Please let me know if:

1. You prefer the API to be part of the bp::opstream/bp::ipstream. So users won't need to go into ip.pipe.set_pipe_capacity().
2. Would you like me to add it to the doc's?
3. When could it be pushed in?

[klemens-morgenstern]

I don't think this should be part of the API. It's linux only , so you can just use fcntl manually. Its trivial, really.