Check for existing S3 bucket using `get` API call

[knowuh]

In lib/shutterbug/storage/s3_storage.rb#creat_bin we should try first using @bin = self.connection.directories.get(Configuration.instance.s3_bin) and checking to see if that returned a valid result, before creating the @bin using

   self.connection.directories.create(
      :key    => Configuration.instance.s3_bin,
      :public => true)

This morning we learned that if we have more than 100 buckets and call directories.create S3 returns a 404, which isn't handled nicely by Fog.

FYI: @scytacki @jengoree