Use a wrapper script for running emsdk.py (#292)

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.

emsdk.bat

@@ -33,7 +33,7 @@
 @SET EMSDK_PY=python
 
 :end
-@call %EMSDK_PY% "%~dp0\emsdk" %*
+@call %EMSDK_PY% "%~dp0\emsdk.py" %*
 
 @set EMSDK_PY=