Add icons to Add dropdown items.

Before:

![Screen_Shot_2015-04-06_at_15.15.44](https://gitlab.com/gitlab-org/gitlab-ce/uploads/a9f7d2e5dbd604fe7dff03cd7e47860e/Screen_Shot_2015-04-06_at_15.15.44.png)

After:

![Screen_Shot_2015-04-06_at_15.15.34](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c5be6cdbe32d1064da7497a1879a9eb3/Screen_Shot_2015-04-06_at_15.15.34.png)

See merge request !498