GenericValue: explicit constructors

In case of overloaded functions taking either a GenericValue or another
class that can also be constructed from the same primitive types
(e.g. std::string, which can be constructed from const char*), the
overloading becomes ambiguous:

  void foo( const std::string& );
  void foo( const rapidjson::Value & );

Declaring the GenericValue constructors taking primitive types as
'explicit' avoids this problem.  This should not have any negative
side-effects, since a GenericValue can't be copied or implicitly
converted to other types.

Fixes http://code.google.com/p/rapidjson/issues/detail?id=70.