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. 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
```