Adds a `Graph#minus` method which will return only unique `VertexId`'s from the calling `VertexRDD`.

To demonstrate a basic example with pseudocode:

```
Set((0L,0),(1L,1)).minus(Set((1L,1),(2L,2)))
> Set((0L,0))
```

Author: Brennon York <brennon.york@capitalone.com>

Closes #5175 from brennonyork/SPARK-6510 and squashes the following commits:

248d5c8 [Brennon York] added minus(VertexRDD[VD]) method to avoid createUsingIndex and updated the mask operations to simplify with andNot call
3fb7cce [Brennon York] updated graphx doc to reflect the addition of minus method
6575d92 [Brennon York] updated mima exclude
aaa030b [Brennon York] completed graph#minus functionality
7227c0f [Brennon York] beginning work on minus functionality