Issue #2: Add support for bugfix branch management

 * Added documentation for git-bugfix.
 * Added tests for git-bugfix.
 * Updated README to include details about git-bugfix.
 * bin/git-branch-prefix
   * prefix_is_valid now checks against the prefix name (was project.prefix).
   * get_prefix now includes the project name since the passed prefix no longer includes the project name.
 * html/git-branch-prefix.1.html
   * Added missing header and footer content.
   * Cleaned up markup.
 * test/features/step_definitions/BranchPrefixSteps.rb
   * Moved "Given I have a working git repository"
   * Moved custom commands to "Given I have a tool to manage the branch prefixes".
   * Removed Before section.
 * Added test/features/step_definitions/RepositorySteps.rb to centralize common repository related steps.