Reloader fixes

There are two somewhat related reloader fixes in this PR:

1. When using "python -m werkzeug.serving module:app" to start an app the reloaded app has the path of serving.py in sys.path[0] instead of the current directory. This needs to be fixed to allow the module where the app is to be found.

2. Inspired by the Flask/Click integration I have added support for lazy app loading to run_simple. This is achieved by passing the app to run as a "module:app" string.

Let me know what you think.

Miguel
You could, sure. But there is going to be a change in behavior. I did it this way because I wanted any errors resulting from importing the application to be reported immediately when the reloader is not used. If you always use this new ImportedAppMiddleware then even w/o using the reloader you are going to have to wait until the first request is sent to find any errors. I think I prefer errors to be reported immediately when the reloader is off, but I don't feel super strongly about this, can change it if that's the preferred behavior.