Appending new line to combined js files

I ran into an issue where one javascript file contained comments on the last line:
```javascript
}]; // some comments
```
and the following file contained comments on the first line(s):
```javascript
/*
 * This is some header text
 */
```
while ended up causing exceptions in the resulting merged version:
```javascript
}]; // some comments /*
 * This is some header text
 */
```

This PR simply ensures there's a newline separating each file, however this might break minification depending on which order they occur (which I'm not sure of), however I would assume that minification would occur *after* combining.