Use a wrapper script for running emsdk.py

This change moves the python code for emsdk into a file ending in .py.
This script is then run via emsdk.bat on windows or emsdk (a shell
script) on non-windows.

This avoid the #!/bin/sh at the top of the python script and the "exec"
hack on the first line that re-runs it under python.  Hopefully this
preserves the intent of #273 without jumping through so many hoops.

.flake8

@@ -1,3 +1,3 @@
 [flake8]
 ignore = E111,E114,E501,E261,E266,E121,E402,E241,E701
-filename = emsdk
+filename = emsdk.py