Reserved activity attributes should start with an underscore

[cressie176]

Currently reserved activity attributes may clash with user defined ones. This makes it really difficult to add new reserved activity attributes in future.

We should change all reserved activity attributes to start with an underscore. e.g.

        {
            _task: 'set',
            name: 'foo',
            value: 'bar'
        }

And prevent users from adding custom attributes that start with an underscore