all: rename Tlv namespace to tlv for consistency

This commit renames the Tlv namespace defined in
tlv.hpp to tlv to be more consistent with tlv::security
and tlv::nfd. A Tlv namespace alias is provided for
backwards compatibility.

Change-Id: I21278d6a077afe7776802c3296997d4c1c44c6c6
diff --git a/src/security/signature-sha256-with-rsa.cpp b/src/security/signature-sha256-with-rsa.cpp
index 3c84b54..cda06e6 100644
--- a/src/security/signature-sha256-with-rsa.cpp
+++ b/src/security/signature-sha256-with-rsa.cpp
@@ -24,14 +24,14 @@
 namespace ndn {
 
 SignatureSha256WithRsa::SignatureSha256WithRsa(const KeyLocator& keyLocator)
-  : Signature(SignatureInfo(Tlv::SignatureSha256WithRsa, keyLocator))
+  : Signature(SignatureInfo(tlv::SignatureSha256WithRsa, keyLocator))
 {
 }
 
 SignatureSha256WithRsa::SignatureSha256WithRsa(const Signature& signature)
   : Signature(signature)
 {
-  if (getType() != Tlv::SignatureSha256WithRsa)
+  if (getType() != tlv::SignatureSha256WithRsa)
     throw Error("Incorrect signature type");
 
   if (!hasKeyLocator()) {