Issue #3: Add support for feature branch management

 * Resolves #3.
 * Added documentation for git-feature.
 * Added tests for git-feature.
 * Updated README to include details about git-feature.
 * Removed test/features/Bugfix.feature as it is now covered by test/features/IssueBranch.feature.
 * Removed test/features/step_definitions/BugfixSteps.rb as it is now covered by test/features/step_definitions/IssueBranchSteps.rb.