Support for Promises, (async/await)

**_keygen_** function is normal function not Promise or async function. I confused for a while and realized it may want some async methods.

code
```
keygen({
location: location,
comment: comment,
password: password,
read: true,
format: format
}, function(err, out){
console.log('111')
fs.writeFile('aaa',out.key)
});
console.log('222')

```
result
```
222
111
```