Remote file task

The commit introduces a `remote_file` task, allowing the existence of a remote
file to be set as a prerequisite.  These tasks can in turn depend on local
files if required. In this implementation, the fact that we're dealing with a
file in the shared path is assumed.

As as example, this task can be used to ensure that files to be linked exist
before running the `check:linked_files` task:

      namespace :deploy do
        namespace :check do
          task :linked_files => 'config/newrelic.yml'
        end
      end

      remote_file 'config/newrelic.yml' => '/tmp/newrelic.yml', roles: :app

      file '/tmp/newrelic.yml' do |t|
        sh "curl -o #{t.name} https://rpm.newrelic.com/accounts/xx/newrelic.yml"
      end