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
I did not know `__loader__` existed, but it seems the logic will not be trivial. You have to check if it exists or not, and also it is different in Python 2 vs. 3. Are you sure it makes sense to go with something like this?

I agree my solution with sys.path[0] isn't great either. Would it make more sense to pickle the whole sys.path and restore it on the reloaded app?