Add Poly1305 authenticator algorithm (RFC 7539)

Test vectors are included from RFC 7539.

Poly1305 is also added to the benchmark program.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 4c8fc3c..7d0960a 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -283,6 +283,7 @@
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT
 //#define MBEDTLS_MD5_ALT
+//#define MBEDTLS_POLY1305_ALT
 //#define MBEDTLS_RIPEMD160_ALT
 //#define MBEDTLS_RSA_ALT
 //#define MBEDTLS_SHA1_ALT
@@ -2399,6 +2400,15 @@
 #define MBEDTLS_PLATFORM_C
 
 /**
+ * \def MBEDTLS_POLY1305_C
+ *
+ * Enable the Poly1305 MAC algorithm.
+ *
+ * Module:  library/poly1305.c
+ */
+#define MBEDTLS_POLY1305_C
+
+/**
  * \def MBEDTLS_RIPEMD160_C
  *
  * Enable the RIPEMD-160 hash algorithm.