Refactor Target.java and add tons of tests

Make Target an interface implemented by SrvTarget and ExplicitTarget.
This allows us to not collapse multiple master endpoints into one
target, which will enable better CLI output later on.

E.g. `helios jobs --master master1 --master master2 --json` outputs

```
{
  master1: {A},
  master2: {B}
}
```

instead of

```
{
  A+B
}
```
@rculbertson How can I make the returned SrvTarget object play nice with the return type of this function?