Implement RDS support

[arut]

Add mysql_format [plain|rds] to config with default plain.

plain is the current behaivior,
rds can be filtered with HttpRdsJsonModule HttpRdsCsvModule
(details here http://wiki.nginx.org/HttpDrizzleModule#Output_Format)

[c0de9en]

Good day.
Is it possible to implement it? I need to make queries to mysql from nginx and convert results to json. The drizzle module requires too many junk packages to compile. Your module works fine but I must make json "by hands".