General tidy and refactoring:
* Use nullptr (C++11) instead of NULL
* Put each file into its own namespace declaration
* Remove "using namespace beast" and qualify ids with "beast::"
* 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