Issue #4: Add support for hotfix branch management

* Resolves #4.
* Added documentation for git-hotfix.
* Added tests for git-hotfix.
* Added command for checking out the master branch.
* Updated README to include details about git-hotfix.
* Moved "Given the custom git config has the default branch prefixes" from test/features/step_definitions/IssueBranchSteps.rb to test/features/step_definitions/BranchPrefixSteps.rb.
* Moved "Given I am on the branch I want the <type> created from" from test/features/step_definitions/IssueBranchSteps.rb to test/features/step_definitions/BranchSteps.rb.
* Moved "Then I should have the <type> branch checked out into the working tree" from test/features/step_definitions/IssueBranchSteps.rb to test/features/step_definitions/BranchSteps.rb