Adds username as data attribute instead of parsing it from the URL

The autocomplete feature was parsing each username from the link found from the HTML output. The links were removed in Elgg 1.9.6 so the feature got broken. Now the mentions javascript adds the username as data attribute so it can be easily accessed later when selecting a user.

Fixes #24
We could map the usernames to an array. When an autocomplete item is clicked, we count that it's the nth ```<li>``` in the list and then we get the corresponding username from the array.