Added minimum to max issues number field

This means that the user can't select a number less than zero, e.g. -1, by using the picker built into compliant browsers.

Additionally, I've made sure to display the current values of settings that are already set. This is what users show page now looks like:

![screen shot 2014-11-13 at 19 06 16](https://cloud.githubusercontent.com/assets/10128/5039716/53534014-6b68-11e4-97ae-e9e310c7621e.png)