Implement GoRename (closes #187)

This PR is going to integrate `gorename` (https://groups.google.com/forum/#!topic/golang-nuts/96hGPXYfqsM)  into vim-go. Currently it's working quite fine. Docs and some other UX things are missing yet. After testing I'm going to merge it.

To test just put your cursor under an identifier and call:

```vim
:GoRename
```

which will prompt you to enter the desired result. Or you can entirely skip the prompt by passing the desired identifier:

```vim
:GoRename toMyIdentifier
```