intervals created by setInterval remain uncleared when manually invoking next and or previous

I've found that intervals are cleanly cleared when invoking next & previous, by always clearing existing intervals in "cycle" before setting up a new interval, the problems that result from multiple intervals go away:

Here's an example: 

```
setInterval 24 bootstrap-carousel.js:44
setInterval 25 bootstrap-carousel.js:44
clearInterval 25 bootstrap-carousel.js:75
setInterval 26 bootstrap-carousel.js:44
setInterval 28 bootstrap-carousel.js:44
clearInterval 28 bootstrap-carousel.js:75
setInterval 29
```
In the above, the interval 24 was just created, but gets lost and not cleared.