Added the principal logarithm.

refs.bib

@@ -111,4 +111,14 @@
   number={10},
   pages={3211--3212},
   year={1996}
-}
+}
+@book{ConwayComplex,
+  title={Functions of One Complex Variable I},
+  author={Conway, J.B.},
+  isbn={9780387903286},
+  lccn={lc78018836},
+  series={Functions of one complex variable / John B. Conway},
+  url={https://books.google.ca/books?id=9LtfZr1snG0C},
+  year={1978},
+  publisher={Springer}
+}