Expose Middleware

This exposes the Express middleware on the module.

Use: `app.use( bitauth.middleware );`
actually according to gnu.org regarding expat license:
```
This is a lax, permissive non-copyleft free software license, compatible with the GNU GPL. It is sometimes ambiguously referred to as the MIT License.

For substantial programs it is better to use the Apache 2.0 license since it blocks patent treachery.
```