Purger purges dates only for "post" type posts

There is a bug in purger.php ad line 144:

``` php
if ( $_post_type == 'post' ) {
...
```

dates are being purger only for "post" type posts I think this should be:

``` php
if ( in_array($_post_type, get_post_types('', 'names')) ) {
...
```

now it will work also for custom post types.
