Fix next backlog date

Fixes next backlog date in API which currently uses 7 day cycle time regardless of config, Also changes display on UI as below:
![before](https://cloud.githubusercontent.com/assets/3397069/3423696/e46f0fda-ffa5-11e3-9b5f-f26ea72b6a49.PNG)
![after](https://cloud.githubusercontent.com/assets/3397069/3423697/e771211e-ffa5-11e3-8269-31a301ab460b.PNG)

I haven't altered the API to include a time as I wanted to ensure it didn't want it to break the compatibility with apps using the API - if you feel there is a low enough risk of it breaking anything I'm happy to make this alteration.