Added Setting constructor taking a std::string.

To address hyperrealm/libconfig#2, a constructor has been added to the
header and source file.
Removed cast from Setting to const char*.

To address hyperrealm/libconfig#1, the cast operator from Setting to
const char* has been removed. As a result, lookupValue() taking a const
char* has also been removed as it uses the cast operator.

This change allows code that assigns directly to a std::string or a
const char* to compile without errors. No change to existing code should
be required.