Return cart item messages as key => value array rather than HTML.

In the admin when viewing a purchase log item with personalisation, if the customer has entered line break into the field, these are not displayed.

Also, as $messages contains an HTML line break that is later escaped using esc_html() in purchase_log_custom_fields() which outputs the HTML as text.

I did a search of the codebase and wpsc_purchlogs_custommessages() is only used here. My feeling is this should return an array of data rather than HTML - keep everything as data for as long as possible and format it just before output.

This commit alters wpsc_purchlogs_custommessages() to return an array, and formats the output of purchase_log_custom_fields() with line breaks.