Use rsync instead of cp when activating versions

[jasonmorganson]

The cp command is not capable of handling symlinks properly. The rync command treats symlinked directories on receiver as regular directories.

This is a way of working around the issues cp reports when it encounters symlinks as reported in Issue #100.

[benatkin]

I like rsync but I'd rather not make it a dependency.

[benatkin]

I'm debating whether to make it detect whether it needs to use rsync and use rsync, or to run cp more than once.

[tjwebb]

I also like rsync, but I'm worried about it causing many of the issues that n's curl dependency has caused.