[WIP] Try to preserve supplied options.async - #184

Hi there,

Say, I ran this code:

var sh = require('shelljs');

sh.exec("sleep 0.5", {async: false}, function() {
    sh.echo("print first");
});

sh.echo("print last");

...as output, I will get:

print last
print first

Your documentation on shelljs.org seems to imply that if you pass in an {async:false} as options, you still get to run code synchronously.

Cheers. M.