Fix setting time_format that was no effect.

time_format option has no effect.
Time#parse will be called to parse time string.
(see https://github.com/fluent/fluentd/blob/v0.10.41/lib/fluent/parser.rb#L30 )

Time#parse will fail in some cases. (e.g. '12-24 10:20:30' no years)
Please check adding tests.

time_format option of RegexpParser is enable to call initialize method with conf argument.
(see https://github.com/fluent/fluentd/blob/v0.10.41/lib/fluent/parser.rb#L66 )