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. 
Granted, changed the DateFormat in the middle isn't all that pretty but it was an attempt to reuse the same DateFormatter for both dates.

So you're imagening something along the lines of 

```
+ (NSDateFormatter *)dateFormatter {
   static NSDateFormatter *dateFormatter;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
      dateFormatter = [[NSDateFormatter alloc] init];
      dateFormatter.dateFormat = @"d";
   }
   return dateFormatter;
}


+ (NSDateFormatter *)accessibilityDateFormatter {
   static NSDateFormatter *dateFormatter;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
      dateFormatter = [[NSDateFormatter alloc] init];
      [dateFormatter setDateStyle:NSDateFormatterLongStyle];
      [dateFormatter setTimeStyle:NSDateFormatterNoStyle];
   }
   return dateFormatter;
}
```

And then two formatDate(ish) methods that uses the two different dateFormatters and returns different strings?

Or perhaps just put the dispatch_once part in each of the formatDate methods since this is the only place it is called.

If that suits you better I can give it a shot if you'd like me to?