add Execute static class

Execute provides Now and Async methods. Now will create and execute a task
in one step and it will execute immediately. Async creates and executes
asyncronously.

All tests were refactored to use Execute.Now. Along the way, a ReSharper
settings file was added.
remove Execute.Async

Don't want this on master yet. It needs to be combined with the other
upcoming PRs and tested with something like Nancy or MVC async
controllers.