General tidy and refactoring:

* Use nullptr (C++11) instead of NULL.
* Put each file into its own namespace declaration.
* Remove "using namespace" directives and add scope qualifiers.
* Control when beast's implementation of std::equal (C++14) is used.
* Tidy up some const declarations.

Conflicts:
	src/ripple_app/shamap/SHAMapSync.cpp
	src/ripple_app/tx/TransactionEngine.cpp

modules/beast_core/text/Identifier.cpp

@@ -21,6 +21,9 @@
 */
 //==============================================================================
 
+namespace beast
+{
+
 StringPool& Identifier::getPool()
 {
     static StringPool pool;
@@ -67,3 +70,5 @@ bool Identifier::isValidIdentifier (const String& possibleIdentifier) noexcept
     return possibleIdentifier.isNotEmpty()
             && possibleIdentifier.containsOnly ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_-:#@$%");
 }
+
+}  // namespace beast