Added default mapping blocks

• Added WANSCodingStore to the main header
• Added a way to register default mapping block for specific classes.
  For example, you can now add a default mapping block to turn strings to NSDate

id(^toDateMappingBlock)(id ) = ^id(id value) {
    if ([value isKindOfClass:[NSString class]]) {
        return [defaultDateFormatter dateFromString:value];
    }

    return value;
};

[mapper addDefaultMappingBlock:toDateMappingBlock
           forDestinationClass:[NSDate class]];