Support for MAX31855 thermocouple

Support for the MAX31855 thermocouple, which uses SPI communication.

A new configuration parameter "sensor" is added to specify which type of temperature sensor to use: "thermistor" or "max31855". The default value is "thermistor", which makes the configuration format backwards compatible.

The implementation of temperature sensors is separated from the TemperatureControl class using a generic  TempSensor interface.

As a future improvement, the TempSensor could be made a full Smoothie module to be used tasks other than temperature control. However, this would add some complexity and break backwards compatibility. 

Comments are welcome.
/Håkan