Add FOG-based S3 input plugin

This is essentially a clone of LogStash's AWS S3 input plugin, but it has been modified to use the FOG library instead of AWS-SDK, which in my case was dying of mysterious timeout exceptions. 

It requires the fog and unf gems.

Sample input section:
input {
   s3fog {
    bucket => 's3-bucket-name'
    credentials => "credential_file.conf"  # see S3 plugin for more info
    region => 'us-east-1'
    region_endpoint => 'us-east-1'
    backup_to_bucket => 's3-backup-bucket-name'
    backup_endpoint => 'us-east-1'
    prefix => 'foo'   # logfile path/prefix
    interval => 60
    delete => false
    debug => false
  }
}