Implement cursortype

A rework of the `cursortype` option. 

Considering [`_testing/test.0009/test.go`](https://github.com/nsf/gocode/blob/021f6a9734f494dd31cd0655f43ca6369a98f106/_testing/test.0009/test.go)

Cursor position `123` corresponds to the `e` of `Mutex` [on line 12](https://github.com/nsf/gocode/blob/021f6a9734f494dd31cd0655f43ca6369a98f106/_testing/test.0009/test.go#L12)

```bash
$ gocode -in _testing/test.0009/test.go cursortype 123
sync.Mutex,,/home/myitcv/.gvm/gos/go1.2.1/pkg/linux_amd64/sync.a
```