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/signature-info.hpp b/src/signature-info.hpp
index 4ef60fc..5dcdcff 100644
--- a/src/signature-info.hpp
+++ b/src/signature-info.hpp
@@ -31,12 +31,12 @@
 class SignatureInfo
 {
 public:
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : Tlv::Error(what)
+      : tlv::Error(what)
     {
     }
   };
@@ -44,21 +44,21 @@
   SignatureInfo();
 
   explicit
-  SignatureInfo(Tlv::SignatureTypeValue type);
+  SignatureInfo(tlv::SignatureTypeValue type);
 
-  SignatureInfo(Tlv::SignatureTypeValue type, const KeyLocator& keyLocator);
+  SignatureInfo(tlv::SignatureTypeValue type, const KeyLocator& keyLocator);
 
   /**
    * @brief Generate SignatureInfo from a block
    *
-   * @throws Tlv::Error if supplied block is not formatted correctly
+   * @throws tlv::Error if supplied block is not formatted correctly
    */
   explicit
   SignatureInfo(const Block& block);
 
   /// @brief Set SignatureType
   void
-  setSignatureType(Tlv::SignatureTypeValue type);
+  setSignatureType(tlv::SignatureTypeValue type);
 
   /// @brief Get SignatureType
   int32_t