The Blizzard Lua API IsAddOnLoaded is depreciated and going away when The War Within launches. You use the API in the files located in the style folder.

Assuming you want backwards compatibility with Classic (Wrath) and Classic Era (Seasons, hardcore, and Classic Era), you need to add this line after creating the ns variable:

local ns = (select(2, ...))
local IsAddOnLoaded = C_AddOns and C_AddOns.IsAddOnLoaded or IsAddOnLoaded

-- the rest of the file here

See this URL for the API changes.

Thanks for your nicely code review!