add __main__.py so "python -m allmydata" works

Thanks to dstufft for the suggestion. I know this can make it slightly
easier to run tahoe in some funny environments (where an appropriate
"python" is on your path but the generated "tahoe" executable is not).

src/allmydata/__main__.py

@@ -0,0 +1,6 @@
+import sys
+
+from allmydata.scripts.runner import run
+
+if __name__ == "__main__":
+    sys.exit(run())