Version 2.0

• Move all json objects to namespace format::json. E.g format::json::number.
• Build target name is now format-json.
• Add const accessors, const & value::at[const wchar * const key] const.
• Store numeric value in long double.
• String, number, boolean are now immutable objects.
• Rename value::count () to size ().
• Add int, long long and float overloads to number.
• Make casting json object to primitive value more accurate.
  Value is only casted to primitive if it is actually possible by static_cast.
  Otherwise json_conversion_error is thrown.
  This also fixes numeric value bug for string type object.
  See: #2
• Object and array iterators are now const iterators, object::const_iterator
• String and number do not lazy generate anything, so string and number::value () are truly const..
• value::stringify () is const.
• value::stringify () returns pointer to new memory.
• Primitive values are stored in union { bool, const wchar_t *, long double }