Fix for #135 <effective> should not come before <expires>

Issue's next time and date are not calculated automatically anymore

![captura de pantalla 2014-06-19 a la s 15 43 13](https://cloud.githubusercontent.com/assets/7064389/3333473/ff909112-f7f3-11e3-8b61-8acb536e47a0.png)


Added validation to check whether expires is before effective

![captura de pantalla 2014-06-19 a la s 15 45 05](https://cloud.githubusercontent.com/assets/7064389/3333475/0824530e-f7f4-11e3-97ad-f78caf5489cc.png)