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/tools/tlvdump.cpp b/tools/tlvdump.cpp
index e66482b..0273861 100644
--- a/tools/tlvdump.cpp
+++ b/tools/tlvdump.cpp
@@ -94,10 +94,10 @@
   std::cout << " (size: " << block.value_size() << ")";
 
   try {
-    // if (block.type() != ndn::Tlv::Content && block.type() != ndn::Tlv::SignatureValue)
+    // if (block.type() != ndn::tlv::Content && block.type() != ndn::tlv::SignatureValue)
     block.parse();
   }
-  catch (ndn::Tlv::Error& e) {
+  catch (ndn::tlv::Error& e) {
     // pass (e.g., leaf block reached)
 
     // @todo: Figure how to deterministically figure out that value is not recursive TLV block