Add start_reading_head cofing parameter to in_tail input plguin

Related #154

(1) Add start_reading_head parameter option to in_tail input plugin.
A new behavior:
in_tail starts reading the head of a log file if pos file exists and fluentd starts first time.
An old behavior:
in_tail starts reading the tail of a log file if pos file exists and fluentd starts first time.

in_tail work like a new behavior if start_reading_head is true.
in_tail work like a old behavior if start_reading_head is fasle.
By default, start_reading_head is false

(2) Fix InputTestDriver
InputTestDriver overrides Engine.emit_stream for tests before input plguin starts.

Before fix:
Override Engine.emit_stream for tests after in_tail input plguin starts.
When input plugin reads logs which exists before it starts,  it call Engine#stream_emit but InputTestDriver does not  still override Engine#stream_emit.