Implement EqualIgnoreOrder

[swelly127]

Hey I work at Dropbox, I think this function would be helpful for a lot of people who need to test equalness in proto objects when they don't care about order of elements in slices.

[awalterschulze]

The first problem here is that gogoprotobuf does not fully support proto.Equal
#13
It has not been tested enough and I am sure there are edge cases, which don't work.
I recommend generating an Equal method using gogoprotobuf's extensions or you are welcome to help me fix proto.Equal by adding more tests.

The second problem is that this code is part of the fork with golang/protobuf. I don't suspect they will go for such a pull request, which means I will have to support this diff and with every merge from golang/protobuf. That means that I will want this feature to be something that a lot of people want.

Maybe a simpler way to solve this is to write a function that uses reflect and recursively sorts all fields that are slices and then does a reflect.DeepEqual after that?
Or if this is only for a specific case, maybe you don't even need to use reflect.

[awalterschulze]

I am closing this, but just comment if you would like it reopened.