core: copy all sub-directories in a box properly

This avoids an Errno::EACCES on windows, although I'm really unsure why.
core: disable parallelization if only one action
website/www: use absolute links in the feed

Some feed readers, like FeedBurner[1] don't support content
that uses relative links. This just makes them absolute so they
will work everywhere.

[1]: https://support.google.com/feedburner/answer/79018?hl=en
Merge pull request #2996 from pearkes/website-rss-absolute

website/www: use absolute links in the feed
kernel/v2: fix error where options could be not hash
Depend on ChildProcess 0.5.0, fix bugs