Fix interval equality

Hi there again :)
I found that the interval equal function doesn't quite work.

For example:
```
teoria.interval("P5").equal(teoria.interval("P8"))
=> true
```

Is there a reason not to compare this.coord with interval.coord directly?
The problem with using sum is that if the result of sub(this.coord, interval.coord) were [n,-n], it will still return true. 
Let me know what you think!

Sean