New theme for vi-mode users: syl20bnr.zsh-theme (based on juanghurtado theme)

This pull request adds a new themes aimed to vi-mode users.

![syl20bnr_01](https://f.cloud.github.com/assets/1243537/67849/6d6dde9a-5f1d-11e2-9c15-d2ad1a1f6039.png)
![syl20bnr_02](https://f.cloud.github.com/assets/1243537/67850/722cf6fa-5f1d-11e2-8120-7f88ee740cc4.png)