Fixed bi-sim to work with std::string as well. Added some necessary stuff to string_impl.h. Removed static define looking for UCI string.
Modified CMakeLists.txt to make the string implementation configurable on the command line. Renamed the macro that governs it.
Clients can now add to ScoringMethods.

ScoringMethods has now public push_back method. Scorer has virtual default dtor, since we don't know if client defined
Scorers will bring their own data members. Added some documentation. C++11 explicit delete for ScoringMethods copy ctor
and assignment operator. Scorer ctor uses delegation.