Error checking on strings received over Serial

[elcojacobs]

The strings that are received over serial for setpoints and other settings are not error checked.

atol just returns 0 on error, which is indistinguishable from atol("0").

By using strtol instead, we can check the end pointer after the function and handle parse errors.

[elcojacobs]

This is fixed in: https://github.com/BrewPi/firmware/compare/cb4ae685101d17adde08fda7942d373f170f55a9...8807f2efe1d39e467bc8123e7919e948e709d297

String conversion functions now return bool and take a pointer to temperature as argument. The temperature is not written if the string is invalid, avoiding the need to write to an intermediate value:

if(stringToTempDiff(target, value)){
    eepromManager.storeTempConstantsAndSettings(); // value parsed correctly
}

Also introduced DISABLED_TEMP in addition to invalid temp to indicate a disabled setpoint independent of mode.

[elcojacobs]

Merged in 0.2.9