Fix Formatting AM/PM for 12PM ({Date,{12,0,0}})

Hey guys,

I came across an issue where ec_date:format will return AM if the hour component of the date is 12.

For example:

```
> ec_date:format("a",{{2013,4,23},{12,0,0}}).
"am"
```

Obviously, this is wrong, so this quick fix solves this, and adds a few tests to go along with it.

Thanks!