Feature request: support store_as gzip

address #16
This patch adds support for gzip-compression of data before storing it.
Yes, only gzip.

In my ssl+kerberos+httpfs environment, this patch works fine.
This configuration parameter name should be same with `compress` of `out_file` for consistency.
https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/out_file.rb#L29