Add support for Laravel's URL and link helper functions

I wrote two Smarty plugins for [Laravel's URL and link helper functions](http://laravel.com/docs/helpers#urls), `{url}` for `action()`, `route()`, `asset()` and `url()` and `{link_to}` for `link_to()`, `link_to_asset()`, `link_to_route()` and `link_to_action()`.

##### Usage Examples

In all cases the `assign` parameter can be used to assign the return value to a given template variable instead of outputting it.

###### {url}

    {* for url() *}
    {url url="…" some_param="…"}

    {* for url() (secure) *}
    {url url="…" some_param="…" secure=true}
    
    {* for asset() *}
    {url asset="…"}
    
    {* for asset() (secure) *}
    {url asset="…" secure=true}
    
    {* for route() and action() *}
    {url route="…"  some_param="…"}
    {url action="…" some_param="…"}

###### {link_to}

    {* for link_to() and link_to_asset() *}
    {link_to url="…" _title="Tag Content" some_param="…"}
    {link_to asset="…" _title="Tag Content" some_param="…"}
    
    {* for link_to() and link_to_asset() (secure) *}
    {link_to url="…" _title="Tag Content" some_param="…" secure=true}
    {link_to asset="…" _title="Tag Content" some_param="…" secure=true}
    
    {* for link_to_route() and link_to_action() *}
    {link_to route="…" _title="Tag Content" param_some_param="…" attrib_some_attribute="…"}
    {link_to action="…" _title="Tag Content" param_some_param="…" attrib_some_attribute="…"}