added full date as accessibility label to PDTSimpleCalendarViewCell

I've added the full date as an accessibilityLabel to the individual dates in your calendar. 

This means that when you tap a date with VoiceOver enabled, instead of just reading the date of the month (eg. 29), the ever so friendly VoiceOver assistent now reads October 29, 2014.

I've taken the liberty of extracting the creation of a DateFormatter to a separate method, which is then used from both the old
```
formatDate: withCalendar:
```
method, and from the new
```
accessibilityFriendlyStringFromDate: withCalendar:
```
Having set the DateFormatter as these methods need, they both call the new (extracted) method

```
stringFromDate: withDateFormatter: withCalendar:
```
Accessibility is enabled for the dayLabel in the
```
initWithFrame: 
```
method and finally, the accessibilityLabel is populated in
```
setDate: calendar:
```
And that's pretty much it. 
Do you have  to set this to YES, or simply  setting the accessibilityLabel is enough?