Added async to ClientProxy

Turns out I don't need to learn macros quite yet... from what I can gather, the futurize function inside of the macro already effectively solves passing a future, so we can just reuse the macro as is.

Creating an async function and using it in the test code appears to work.

I did attempt to rename async to apply, and encountered a type inference problem:
```
autowire/Tests.scala:69: missing parameter type for expanded function ((x$9) => x$9.sloww(Seq("omgomg", "wtf")))
[error]       val res5 = await(Client[Api](_.sloww(Seq("omgomg", "wtf"))))
```

Changing the line to the following fixes the issue and it does run...
```
val res5 = await(Client[Api]((_:Api).sloww(Seq("omgomg", "wtf"))))
```
But I had to add the :Api annotation to all the other uses of Client[Api]...

I have no idea why the compiler complains about inferring the type of _ in this case. Therefore, I have currently left in the async instead of trying to add another apply method.