Lowering min temp to 10C

[Firebie]

Please try and fill out this template where possible, not all fields are required and can be removed.

• Please check if the PR fulfills these requirements

• The commit message makes sense
• [] The changes have been tested locally
• [] Are there any breaking changes

• What kind of change does this PR introduce?
  (Bug fix, feature, docs update, ...)
  Lowering min temp from 50C to 10C

• What is the current behavior?
  (You can also link to an open issue here)
  Question / Feature request: lowest min temp #620

• What is the new behavior (if this is a feature change)?
  User can select 10C as min temp for soldering/sleep

• Does this PR introduce a breaking change?
  (What changes might users need to make in their application due to this PR?)

• Other information:

[Ralim]

Is this stable?
Originally the reason i limited the minimum to 50C was that the PID was unstable below that temperature when running off 24V input.

[Firebie]

Checked 30C - works fine (although current weather is 28).
Checked 40C - works fine.
Power supply - 24V.

[Ralim]

Ah sorry, didn't merge this before i did the other one. Could you merge master in and then ill accept this in :)

[Firebie]

I re-based against master