Caesar cipher

Implement the Caesar cipher for an internal puzzle hunt. Given a message and a shift k (0 to 25), move every letter k positions forward in the alphabet, wrapping from 'z' back to 'a' (and 'Z' to 'A'). Preserve each letter's case, and leave digits, spaces, and punctuation unchanged. For example, 'Hello, World!' shifted by 3 becomes 'Khoor, Zruog!'.