Allow changing Group task id to -1

[janaknat]

Is there a reason for not allowing Group to set CPU, task?

From src/lib.rs:

/// A `Group` and its members must all observe the same tasks and cpus; mixing
/// these makes building the `Counter` return an error. Unfortunately, there is
/// no way at present to specify a `Group`'s task and cpu, so you can only use
/// `Group` on the calling task. If this is a problem, please file an issue.