Add Poly1305 authenticator algorithm (RFC 7539)

Test vectors are included from RFC 7539.

Poly1305 is also added to the benchmark program.
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 78bab7f..251b6c6 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -48,6 +48,7 @@
     pkwrite.c
     platform.c
     platform_util.c
+    poly1305.c
     ripemd160.c
     rsa.c
     rsa_internal.c