Select multiple faces one at a time

This is in to issue https://github.com/DynamoDS/Dynamo/issues/1777.

Drag box selection for to select multiple items only works in some situations.  There needs to be nodes for Revit that allow multiple selection by clicking individual elements.  This pull request is just to provide an example.  I am sure there are others that want this, and this is something I need to have right now for faces so I went ahead and made a SelectFaces node.  Below is the rundown of what using it would be like.  

It looks like the other ones with the addition of an "Add" button:
![2014-06-23 dynamo select faces 01](https://cloud.githubusercontent.com/assets/2084232/3365460/7afccd6a-fb2e-11e3-91ac-fd2fdf26b7cd.PNG)

After the initial selection it looks like this:
![2014-06-23 dynamo select faces 03](https://cloud.githubusercontent.com/assets/2084232/3365461/80d1af8a-fb2e-11e3-8d64-cc7a8ece5061.PNG)

Now I want to add faces to the existing selection:
![2014-06-23 dynamo select faces 04](https://cloud.githubusercontent.com/assets/2084232/3365465/98f4ea5a-fb2e-11e3-8a25-6fed715e8dd6.PNG)

Following adding faces the selection in Revit matches, almost.  I gave up.
![2014-06-23 dynamo select faces 05](https://cloud.githubusercontent.com/assets/2084232/3365467/9dcf4480-fb2e-11e3-822f-776ed189be14.PNG)

If you select "Change" the selection is cleared.  

I'm going to create a "Remove" button in the morning.  Basically the scenario is we have to select a lot of faces, and if mistakes are made, we can't start over.  They need to be able to click to add and remove faces without starting over.

cc: @pboyer