is() now works for tuples

You might want to update Page 177 where you define the isTuple template.

``` d
import std.typecons;

template isTuple(T)
{
    static if (is(T tup == Tuple!(InnerTypes), InnerTypes...))  // now works (2.060+ methinks)
        enum isTuple = true;
    else
        enum isTuple = false;
}

void test(T)(T t)
    if (isTuple!T)
{
}

void main()
{
    auto tup = tuple(1, 2);
    test(tup);
}
```
