support store_as gzip
add configuration test for store_as
add sample configuration for store_as gzip
change parameter name: store_as -> compress
validate compress type
extract compress-related methods, generate_path and compress_context
fix a bug where ConfigError needs Fluent:: prefix
add invalid configure test
Merge pull request #23 from shoichikaji/feature/support-store-as-gzip

Feature request: support store_as gzip