server: increment ports if busy

```Python
In [1]: import blaze
In [2]: server = blaze.Server({})

In [3]: server.run(port=6363)
 * Running on http://127.0.0.1:6363/
    Oops, couldn't connect on port 6363.  Is it busy?
 * Running on http://127.0.0.1:6364/
```