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
What if code modifies sys.path before Werkzeug is loaded and the restoring doesn't actually override Werkzeug's directory but something else?

I propose that we solve this problem by modifying ``args`` to be something like ``[sys.executable, '-m', modulename]`` in this case.

``modulename`` can be obtained by doing ``from __main__ import __loader__ as loader; modulename = __loader__.fullname``. I am not sure this should be done though, because ``__loader__`` is hardly documented. See also https://github.com/mitsuhiko/werkzeug/issues/461