How to quantify caffeine content in coffee?
According to a paper published named: "Chemical Test for Caffeine" for the "Hong Kong Chemistry Olympiad for Secondary Schools (2013-14)" (Hyperlink provided below). You can titrate caffeine, with the help of iodine and potassium iodide, and sulfuric acid as the acid. You will have to add an excess of iodine, which means it will be an iodometric backtitration. The iodine will react in two times the number of moles compared to your caffeine.
The only problem I have with this method, is that I cannot found any elaboration on how this reaction is possible on the internet (I haven't tested it myself). If there is someone who knows more about how this reaction if possible, please provide me with more information. 



If someone knows how or why the reaction can take place, please keep me notified about it. I will provide the reaction below:
Reaction:
Reactants --- Caffeine + 2 Iodine + Potassiumiodide + Sulfuric Acid
Products ---- Potassium Bisulflate + Caffeine (in a peculiar complex with the remaining substances, notated like a hydrate. No water present.)

$$\begin{align}\ce{C8H10N4O2 + 2 I2 + KI + H2SO4 &amp;-&gt; C8H10N4O2 . HI . I4 + KHSO4}\\
\ce{\color{red}{I2} + 2 Na2S2O3 &amp;-&gt; 2 NaI + Na2S4O6}\end{align}$$

EDIT 1: This URL should lead to the paper I mentioned (if it works).
Chemical Test for Caffeine
EDIT 2: Another URL provided, this is the source used in the actual paper. The site is in Chinese (if I'm not mistaken), but can be translated via google. For anyone interested: Source used in mentioned paper, from "www.med66.com"