Enable AVF nodes in 0.7

AVF nodes have been implemented, but were not loaded into the library in 0.7. This pull request enables these nodes and implements element binding logic for ``SpatialFieldManager``. 

The key thing is not only serialize the element id of ``SpatialFieldManager`` but also serialize the primitive id that created by ``SpatialFieldManager``, so I added a class ``SpmPrimitiveIdPair``:
```
class SpmPrimitiveIdPair: ISeriazable
{
    ...
    public int SpatialFieldManagerID {get; set;}
    public int PrimitiveID { get; set; }
    ...
}
```
And call ``ElementBinder.GetRawDataFromTrace()``, ``ElementBinder.SetRawDataForTrace()`` to serialize this information. 

Did some manual test to ensure primitive id could be correctly retrieved from trace data when reopen Dynamo and run the graph.

![avf](https://cloud.githubusercontent.com/assets/2600379/3478921/44621ce8-0348-11e4-8439-a75d79335407.png)