null-safe method invocations

I propose null-safe method invocations using the Elvis symbol, aka `?:`.

This would allow code to be written as:

```
let person = dao: findByName("Mr Bean")
let city = person?: address()?: city() orIfNull "n/a"
```

which is arguably more elegant than:

```
let person = dao: findByName("Mr Bean")
var city = "n/a"
if person isnt null {
  let address = person: address()
  if address isnt null {
    city = address: city() ofIfNull "n/a"
  }
}
```

You can test by building Golo from this branch.

Feedback is welcome! I am leaving this pull-request for 24h without further comments.