Async client refactoring

- All actions now available in strongly typed and simple variants
- URL used are now Dispatch Req, having a richer API to construct a [request](http://dispatch.databinder.net/Defining+requests.html) and [Query parameters](http://dispatch.databinder.net/HTTP+methods+and+parameters.html)
- Implicit conversion offered to continue working with String URLs like before

@coderunner @mmimeault