Added commands to reset a file or a complete castle

I've really been loving using Homesick.One thing that I thought was missing was the ability to reset a file easily after trying out some functionality that doesn't work or to reset an entire castle back to HEAD. This PR adds that functionality. I also added code that allows stdout to be captured as a string from system calls, which enabled me to add tests for my new functionality as well as commit and status. I added the ability to pass a message as a second argument to commit, which if passed is then used as the git commit message. This allows the commit method to be tested. Finally, I also added Guard to the gemfile for the project as I like using it but I can easily remove that.

Let me know what you think. This is my first attempted contribution to a Ruby project and I'd love some feedback.