Close tempfile

A lot of temporary files were left on /tmp when the plugin couldn't 
write to s3 for some reason. I think it's safer to explicitly 
ensure a deletion, as written so in the Tempfile#Good practices doc.

http://www.ruby-doc.org/stdlib-1.9.3/libdoc/tempfile/rdoc/Tempfile.html
Merge pull request #15 from ijin/patch-1

Close tempfile
s3_object_key_format accepts hostname, uuid, etc. parameters from fluent-mixin-config-placeholders