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.
`compress` is more straightforward, and I think it is better too.
In b19590a, I changed the parameter name from `store_as` to `compress`.
Actually I don't think fluent-plugin-webhdfs needs to support both..., but if you really think so, I will fix code.