Set Upload times

[MrDrem]

As per part of the discussion for issue #141 , it would be helpful to have the option of fixed upload times.

"support for staggered times should be added" - I'd love for this to be added as an option. I'd love for my uploads to be scheduled for h:02 & h:32 for a couple of reasons.

1. I'd like to move them away from the readings which we know that the WiFi chips increase the temperature on those upload readings.
2. I'd like to have set times so that failed readings don't force me to have to go and press the poke button to get my readings back to the right upload times. I'm trying to tweet off the basis of my readings, I want to do this each hour on the data that I've had over the last hour, and having this set would be a great help.