Add Poly1305 authenticator algorithm (RFC 7539)

Test vectors are included from RFC 7539.

Poly1305 is also added to the benchmark program.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 3821657..82f1554 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -98,6 +98,7 @@
 add_test_suite(pk)
 add_test_suite(pkparse)
 add_test_suite(pkwrite)
+add_test_suite(poly1305)
 add_test_suite(shax)
 add_test_suite(ssl)
 add_test_suite(timing)