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/encoding/block.hpp b/src/encoding/block.hpp
index 17192d8..9c2db13 100644
--- a/src/encoding/block.hpp
+++ b/src/encoding/block.hpp
@@ -51,12 +51,12 @@
   typedef element_container::const_iterator  element_const_iterator;
 
   /// @brief Error that can be thrown from Block
-  class Error : public Tlv::Error
+  class Error : public tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : Tlv::Error(what)
+      : tlv::Error(what)
     {
     }
   };