provide a way to get the WSGI app

we need this for issue #3

The following now already works:

,----
| gunicorn -w4 'pypiserver:app("/home/ralf/packages")'
`----

pypiserver/__init__.py

@@ -1,2 +1,19 @@
 __version_info__ = (0, 5, 1)
 version = __version__ = "0.5.1"
+
+
+def app(root=None,
+        redirect_to_fallback=True,
+        fallback_url="http://pypi.python.org/simple"):
+    import os
+    from pypiserver import core
+    import bottle
+
+    if root is None:
+        root = os.path.expanduser("~/packages")
+    os.listdir(root)
+    core.packages = core.pkgset(root)
+    core.config.redirect_to_fallback = redirect_to_fallback
+    core.config.fallback_url = fallback_url
+    bottle.debug(True)
+    return bottle.default_app()