Add deleteFriends (experimental)

Sets 'type' to 3 on the friends' records, which causes the friends to no longer show up in the Snapchat client. Seems to work similarly to deleteFriend() in that respect.

Mostly just uses the same code as addFriends. Also uses the exact same API action as addFriends, except it just takes an array of usernames instead of the arrays that addFriends passes for each user.