Clean up handling of multiple columns 

If the user defined a mapping as follows:

```
HasMany(x => x.Children)
  .KeyColumns.Add("one") .Unique() .KeyColumns.Add("two");
```

Then the behaviour is to have two column mappings created, each with the unique set to true. This is somewhat counter-intuitive. It would be better to handle multiple columns with something like

```
.KeyColumns.Add("name", c => { optional column mapping });
```
