Server thread

Servers don't have to block

```Python
In [1]: from blaze import Server

In [2]: server = Server({})

In [3]: server.run(separate_thread=True)
Out[3]: <FunctionRunner(Thread-1, started 139788731410176)>
 * Running on http://127.0.0.1:6363/

In [4]: # Control returned
```