Add support for Locking thermostat keypad

[ocrease]

This PR delivers the feature request in #252. It adds two new entities:

• A Lock entity that allows locking and unlocking. Clicking lock in the UI will use whatever pin number was previously used. If you want to set a new pin number, call the lock.lock service and provide a number between 0 and 9999. Numbers less than 1000 are padded with 0 at the beginning, eg 12 becomes 0012. This entity is available both on thermostats and time clocks.
• User Limit - controls the temperature up/down limit when the thermostat is locked. Setting it to 0 will mean that the temperature can't be changed on a locked thermostat unless it is unlocked. Only available on thermostats