…essary file descriptors. Fixes issue #137.
This makes capybara-webkit compatible with DRb (which replaces $stdout with an object that does not support #reopen). Also, by default, popen closes all nonstandard file descriptors in the spawned process, fixing problems related to #132.
This allows one to close all unnecessary file descriptors in the child process so they don't conflict with whatever the app is expecting. It solves some problems with DRb.