"async void" should not be used

This:

``` csharp
async void FooAsync()
{
}
```

Becomes:

``` csharp
async Task FooAsync()
{
}
```

It's safe to assume that other than something looking like void (sender object, T e) where T:EventArgs is a wrong use of async void.

Severity: `Warning`
Diagnostic Id: `CC0077`
Category: `Usage`

@robertorodrigues is working on it.
