support for symlinked restore source directories / advanced rsync config

Hi,

In our setup we have a situation where we have to store our backups in directory that is only accessible via a symlink on the restoring side. The problem with this is that this doesn't work well with the built-in rsync -a command (used in copyblobs.py), which preserves symlinks as symlinks and does not dereference them (because of the rsync -l option, implied by -a). 

To tackle this problem, I first just adapted the code to add extra parameters (in this case --no-l -k) to the rsync command but then realised it would be nicer to just allow the user to pass any wanted extra rsync parameters via the collective.recipe.backup configuration. So I adapted the code in a fork of this project and -after testing and adding a brief description in the readme- issued a pull request because I think it could also be useful for others using this great product...

For us it would be great to just use the next release of this package but if the pull request does not get merged, we will probably just keep using our own fork.

Kind regards, 

Frederic