fixed bug with negative seconds when round option is set to false

Bug description : __2:50__ expected instead of __3:-10__
If round option is set to false, readingTimeMinutes need to be
calculated with `Math.floor()` to avoid negative seconds.

![capture decran 2014-04-04 a 10 04 43](https://cloud.githubusercontent.com/assets/806883/2613068/d7847176-bbcf-11e3-8e34-5f2cc03dbbd6.png)