For 15 years, San Francisco resident Sirron Norris has prided himself on creating murals across The City that inform people from all walks of life about the cultural relevance of neighborhoods. Corporations such as Mitsubishi, Sony and Ricoh Theta have paid Norris to use his murals for commercial purposes. So when the artist received a calendar in the mail in December 2012 from Zephyr Real Estate — the largest independent real estate firm in The…