Use werkzeug make_server for starting & shutdown

[T4rk1n]

Replace the start of the server by calling make_server from werkzeug which can then be used to shutdown the server properly.

[alexcjohnson]

OK maybe a little unlikely, mostly this works great in my testing, but I did find a way to break it. I noticed when I ran two apps, one on the default port and one on port=8051, then app._servers looked like this:

{('127.0.0.1', '8050'): <werkzeug.serving.ThreadedWSGIServer at 0x121b394e0>,
 ('127.0.0.1', 8051): <werkzeug.serving.ThreadedWSGIServer at 0x1226cdde0>}

So then I tried to rerun the second app, but instead of port=8051 I used port='8051', and THAT gave:

OSError: [Errno 48] Address already in use

Simple fix, just coerce port to be an integer instead of a string.

[alexcjohnson]

💃 after considering #105 (comment)

[aiqc]

Does this also address the dreaded #33?