Replaced Short Names in GerritEventContext by DSL methods

Deprecated syntax:
```groovy
job {
    triggers {
        gerrit {
            events {
                ChangeAbandoned
                ChangeMerged
                ChangeRestored
                CommentAdded
                DraftPublished
                PatchsetCreated
                RefUpdated
            }
        }
    }
}
```

New syntax:
```groovy
job {
    triggers {
        gerrit {
            events {
                changeAbandoned()
                changeMerged()
                changeRestored()
                commentAdded()
                draftPublished()
                patchsetCreated()
                refUpdated()
            }
        }
    }
}
```

Replaces #139.