Faster adding of files if they match any of the excluded regexps

[webdevvie]

Please see my post here: https://github.com/box-project/box2/issues/97

Also see: https://github.com/box-project/box2-lib/pull/23 As this version is required to make this PR work :)

Also maybe the config should be added to the build file documentation. I did not want to mess it up. So maybe someone else should do that ;)