hard-coded constants are configurable

[m-mcgowan]

There are some constants that are presently hard-coded that ideally should be configurable under advanced settings:

• minimum time between heating/cooling
• minimum heating/cooling time

(If there are others, please add to the list)

These could be made into settings and added to the settings structure stored in eeprom, and given corresponding settings attributes in the json encoding over serial.

[elcojacobs]

There are configureable in BrewPi 0.4