Publish an array of cursors

New version of #682 with all (I hope) remarks fixed. 
* Added some tests for 'several cursors of one collection published' and 'not a cursor in array
* Added few lines of docuemntation. 
Please feel free to fix documentation or error messages, English is not my native language as you can notice )

``` 
Meteor.publish('multi', function (num) {
    return [ Foo.find({ num: num }), Bar.find({ num: num }) ];
});
```