Implement "purge_before_symlink" deploy attribute

Companion to PR #201.  Would be nice to have full control over `symlinks`, `symlink_before_migrate`, `create_dirs_before_symlink `, and `purge_before_symlink` when making custom deploy recipes.

Also would be nice to override these defaults in apps where we don't need the `logs`, `pids`, `config`, and `system` symlinks created for our deployments.

I have added a default for `purge_before_symlink` consistent with the definition at:
<https://github.com/opscode/chef/blob/469bed/lib/chef/resource/deploy.rb#L67>

I agree to supply this patch under your Apache 2.0 license.