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
Merge pull request #26 from juho-jaakkola/autocomplete

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