Display just the last 20 lines of the console log for long logs

Display a summarised console log for long logs and allow viewing of the full log.
Fixes #88 
This works, but the same effect can be achieved in a simpler way with slices:

```
@property
def console_log_summary(self):
    log = self.console_log.splitlines()
    if len(log) > self.CONSOLE_TAIL_LINES:
        log = log[-self.CONSOLE_TAIL_LINES:]
        log.insert(0, "...")
    return "\n".join(log)
```

It can probably be simplified further still :-)