Refactor zen_href_link (catalog) and zen_catalog_href_link (admin)

 * Fixed the wrong separator in zen_href_link when sid must be passed with static URLs and no other parameters.
 * Updated the code handling sessions (zenid) to use "constant('SID')" instead of only "SID".
 * Consolidated the link "cleanup" code in zen_href_link into one rtrim, one preg_replace, and one str_replace.
 * Refactored connection check in zen_href_link to default to HTTP and issue a WARNING in the debug log if an unknown connection type is specified.
 * Added a notifier to zen_href_link. This should allow observers to "listen" and "act" upon the function being called.
 * Updated the admin function zen_catalog_href_link to more closely match the catalog zen_href_link.

<sub>**Note:** Should the new language constant (used when issuing a WARNING about unknown connection types) go into the main language file or into a new file under extra_definitions? My initial thoughts were in the main language file as they affect every page (and requires one less file load by the web server / PHP). For the purposes of this patch I've made the edits directly to the main language file(s). These may need to be moved and / or changed.</sub>
Updated refactor of zen_href_link (catalog) and zen_catalog_href_link (admin)
 * Updated internal comments and added PHPDocs to both functions. Included a few examples in the PHPDocs to hopefully help future developers understand the use of the functions.
 * Added support for passing $parameters as an array. Mostly a convienence, but hopefully the change also makes the intended use of the paramater more obvious.
 * zen_catalog_href_link now accepts $search_engine_safe, $static, and $use_dir_ws_catalog. This change is to better match the links generated from the catalog when sending emails.
 * Replaced a call in the admin "header.php" to zen_catalog_href_link() with "HTTP_CATALOG_SERVER . DIR_WS_CATALOG". This matches the behavior of the default catalog "header.php" template.
Fixed a misspelling and moved the notifier hook to allow changing from a non-encrypted connection to an encrypted connection.
Fixed adding the separator when switching between secure and non-secure pages.
Merge pull request #41 from lhungil/refactor-href-link

Refactor zen_href_link (catalog) and zen_catalog_href_link (admin)
Merge pull request #55 from zencart/v160

zc 160