New API for raw value (int of 1/16 degree)

* New functions getTemp, rawToCelsius and rawToFahrenheit to support raw 12 bit integer temp values (1/16 of degree Celsius).
* Improved performance of temp conversion with integers arithmetic - http://myarduinotoy.blogspot.co.uk/2013/02/12bit-result-from-ds18s20.html
* No more issues with DS18S20
* Can also work with DS1825
* Fix typos in comments