Added sec:link wrapper taglib method

This is something I have been using in a local fork and decided to try to contribute it back. I found myself doing the following a lot, and my pull request contains a convenience method to encapsulate it. This:
```html
<sec:access controller="myController" action="myAction">
    <g:link controller="myController" action="myAction">My link</g:link>
</sec:access>
```
Becomes:
```html
<sec:link controller="myController" action="myAction">My link</sec:link>
```