Migrating to C++11 - run clang-modernize.

I've run clang-modernize as suggested in #45 (previously named cpp11-migrate).

Changes include:
* adding override keyword on functions,
* replacing NULL with nullptr,
* enabling moves of some constructor parameters,
* replacing explicit type with auto where the type is obvious or unimportant,
* replacing loops over containers with range-based for.

I've also added explicit keyword to some single-param constructors and moved private static variables in RuleConfiguration and RuleSet to implementation files.