Execute class

This adds a new `Execute` static class with a `Now<T>()` method that will create and execute the task in one step. The tests have all been refactored to call `Execute.Now<T>()`. I think it improves readability.

This PR also adds a ReSharper settings file (still a work in progress). I may yank it out of this PR.

Bonus: References to `Simpler.Task` will rarely be needed since `Execute.Now<T>` can be called instead of `Task.New<T>`. This avoids the namespace conflict with `System.Threading.Tasks.Task` and would fix #17 .

Tasks
- [ ] Finalize ReSharper settings