delay the discovery of sensors after powerup

Calling ow_periodic() from onewire_init() only works when
DEBUG_OW_POLLING is enabled. OW_DEBUG_POLL inside ow_periodic()
delays the discovery for some time and the discovery then works.
Without DEBUG_OW_POLLING, the discovery is not being delayed and
fails for some reason.
To solve this, ow_periodic() will be first called after 50*20ms = 1s.

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Merge pull request #213 from mguentner/1w-discovery-fix

delay the discovery of sensors after powerup