Enhancement: Allow user to load incomplete relation members

**Description**
This enhancement is to enable user to incomplete load relation members, shown as ```<not downloaded>``` in ```iD.ui.RawMemberEditor```.
This is a simple change but enhances the user experience greatly, especially in managing relation entities.

By simply calling ```connection.loadEntity(relationId)```, we can fill in the missing related entities (ways / nodes).

![screenshot-01](https://cloud.githubusercontent.com/assets/78585/3547215/a48c8ac8-0892-11e4-90cd-112a38fc45bc.png)

**Demo**
Checkout feature branch [feature:load-incomplete-members](https://github.com/sogko/iD/tree/feature/load-incomplete-members) and edit the following relations to demonstrate typical relation member sizes:

* Singapore's East-West Line (49 relation members)
[http://localhost:8000/#background=Bing&id=r445764&map=20.00/103.90306/1.31981](http://localhost:8000/#background=Bing&id=r445764&map=20.00/103.90306/1.31981)

* Paris' RER B Train Route (352 relation members)
[http://localhost:8000/#background=Bing&id=r65074&map=19.00/2.56124/49.00988](http://localhost:8000/#background=Bing&id=r65074&map=19.00/2.56124/49.00988)


**Discussion**
* Impact on performance
    * According to [OSM Wiki](http://wiki.openstreetmap.org/wiki/Relations), relations are not recommended to have more than 300 members. But of course, contributors may choose to ignore this guideline
* Letting user complete one relation member at a time
    * Instead of loading all of the relation members at one time, another possibility would be to load one at a time
    * But currently, there is no hint as to the full content of the incomplete member. In other words, user would only be shown ```<not downloaded>``` at first and possibly the ```role```. The user would have to load each one of them until he/she finds the member that he/she is looking for.
* Placement of button
    * Currently, the enhancement has the button next to the form-label of a member, which might suggest that the action to be taken would only affect one member.
    * A better placement would be somewhere else where it suggests that the action will affect all members. (Like 'Complete All Members' or something to that affect)


**Notes**
* The buttons are currently styled with an interim icon ('geocode'). 
* jshint OK ```jshint js/id/ui/raw_member_editor.js```