Convert ID Arg into an Array

Allow the ID arg in both the shortcode and the php function to be either an array or integer.

These were tested:
``` php
<?php woothemes_features( array( 'limit' => 3, 'id' => array(13906,13898,'abc','def','     ','who loves pizza!?',13894) ) ); ?>
<?php woothemes_features( array( 'limit' => 3, 'id' => 13906 ) ); ?>
<?php woothemes_features( array( 'limit' => 3, 'id' => "" ) ); ?>
```
```
[woothemes_features limit="3" id="13906,13898,abc,def,    ,who loves pizza!?,13894"]
[woothemes_features limit="3" id="13906"]
[woothemes_features limit="3" id=""]
```

See #24