Split logoutOtherClients into two methods.

One method call gives you a new token, and another method call removes
all but the current token. Meteor.logoutOtherClients calls both these
methods, and calls the user's callback when the second has returned.
whitespace and use 'self' instead of 'this'
Add comments to logoutOtherClients methods
Merge branch 'two-methods-for-logout-others' into devel

Fixes #1915.
Add History entry for `Meteor.logoutOtherClients` callback.