edge modification / dropping

Hello,

I'm using based on #20 the 1.1.2 version, but it seems to be that also modification / dropping support for edges (on the `mode.Edge` struct) is missing. For dropping edges by ID I'm using:

```golang
client.ExecuteStringQuery(fmt.Sprintf("g.E('%v').drop()"))
```
And I would be nice, if the call `AddProperty` could be added to the `model.Edge{}` struct as well.

Thanks