IsDependentOn with multiple tasks

I'd like to be able to use `IsDependentOn(IEnumerable<> tasks)`:

```c#
var tests = new[]
{
Task("Test 1").Does(() =>
{
// ...
}),
Task("Test 2").Does(() =>
{
// ...
}),
Task("Process output containing format string should succeed").Does(() =>
{
// ...
}),
};

var target = Argument("target", "Default");

Task("Default")
.IsDependentOn(tests);

RunTarget(target);
```

Start at the IsDependentOn API and trace how the existing task dependency call handles its arguments. Confirm how the IEnumerable overload should behave with the tasks in the example, then verify that the Default target depends on all three tasks when RunTarget(target) is used.

Written by the indexing model from the issue text.