If method returns a Task it should have the postfix "Async"

This is a convention in .NET, so:

``` csharp
Task Foo() { }
```

Becomes:

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

The `async` keyword is not necessary for the method to have the `Async` postfix.
When renaming the method, all references have to be renamed as well and no code should break.

Category: `Style`
Diagnostic Id: `CC0061`
Severity: `Info`
