Add support for passing function as signInRequired extra condition

Fixes #283.

Basic usage example:

```
onBeforeAction: ->
	AccountsEntry.signInRequired this, ( user ) -> Roles.userIsInRole user._id, 'closed-section'
```

Of course, you can put any access login in this callback:

```
onBeforeAction: ->
	AccountsEntry.signInRequired this, ( user ) ->
		user = User.first user._id
		user.hasClosedSectionAccess() or user.isAdmin()
```