Added tests for commit and status methods

I've added a method to spec_helper.rb that allows the output of a system call to be captured. With this I've been able to add tests for the commit and status methods. I also added an option to commit to pass a commit message as the second argument. Without this it was impossible to test commit as it just opened a dialogue waiting for input. Finally, I also added guard and guard-rspec to the project gemfile for running tests continuously.