Bugfix: emonGLCD send time interval default 0s

I hopes this fix solves the issue encountered by Bra1nK (https://github.com/emoncms/raspberrypi/issues/22).

What we want is the sendtimeinterval field to be created with a value of 0. When it does not have any value, the script crashes.

I'm not sure it will act retro-actively, though, for people who already updated their DB and have a field with no value. If not, users will have to manually set a value via the RaspberryPi tab.

Trystan, please tell me if you think this modification is enough to create sendtimeinterval with a value of 0. I'm just discovering the update process (nice job, BTW). If so, we could close https://github.com/emoncms/raspberrypi/issues/22.