Added leading zero & restrict EAC

Added an option leading zero to timer. This is handy if you want to use the timer as a countdown clock. If the number is <10 it will add a leading zero to it.

I created separate scope variables for the format (ddays, mminutes, sseconds)

```
<div data-timer end-time="1400799600000" add-leading-zero>{{ddays}}:{{hhours}}:{{mminutes}}:{{sseconds}}</div>
```

Updated docs.

I also set restrict to EAC because you should be able to use elements as well as attributes and classes in your markup,