Supported 'timezone' configuration parameter. (e.g. "+09:00" for JST)

This commit adds '**timezone**' configuration parameter which enables users to specify an arbitrary timezone. The following is an example of the usage.

    <match pattern>
      type file
      path /var/log/fluent/myapp
      time_slice_format %Y%m%d
      time_slice_wait 10m
      time_format %Y%m%dT%H%M%S%z
      compress gzip
      timezone +09:00
    </match>

The current implementation of fluentd supports 'localtime' and 'utc'. This commit adds an arbitrary timezone there.

The value format of 'timezone' is the format that Time.localtime(utc_offset) accepts. For example, "+09:00".
Inserting `return nil if timezone.nil?` is better?