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/native/linux_Files.cpp

@@ -21,6 +21,9 @@
 */
 //==============================================================================
 
+namespace beast
+{
+
 enum
 {
     U_ISOFS_SUPER_MAGIC = 0x9660,   // linux/iso_fs.h
@@ -367,3 +370,5 @@ void File::revealToUser() const
     else if (getParentDirectory().exists())
         getParentDirectory().startAsProcess();
 }
+
+}  // namespace beast