* Default Python 3 for running emsdk
On macOS it's pretty common to hit a problem
```
Error downloading URL 'https://github.com/kripken/emscripten/archive/1.38.25.tar.gz': <urlopen error [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:590)>
```
When an installation is started. I.e. `./emsdk install sdk-1.38.25-64bit` - this is caused in majority of cases by some problem of openssl and python2 - (some other https downloads work just fine, not sure why GitHub is special)
So this Pull Request will default Python to Python 3, for running `emsdk`. It does so by making ./emsdk executable in python, bash, and sh. In bash and sh it will run a python executor script, which picks the best python it can find, preferring python3 on non-windows.
