Execution

Just for feedback
Will add scaladoc.

I don't prefer the approach, as such, but the type of continueWithStats does encapsulate a key piece of knowledge: that if you have continued with the load, then your LoadInfo is a LoadSuccess, and you can read the stats fields accordingly without having to do any casts.

To see the justification for this, take a look at the new CustomerSqoopImportExecution example, compared to the old code. We used to have to write:

```
if loadInfo.continue
loadCount = loadInfo.asInstanceOf[LoadSuccess].written // ouch!
```

Whereas with this method you can replace those two lines with:

```
loadSuccess        <- loadInfo.continueWithStats
```