change gush git-remote configuration handling

- each remote (except origin) corresponds the org
- provide a way to get the fork-origin to better guess correct org
- move git configuration handling to its own helper
update PullRequestMergeCommand for new GitConfigHelper
add missing file StringUtil.php
fix broken tests
fix phpdoc-typo
Merge pull request #359 from sstok/347-better-repository-information-providing

improve git-remote configuration handling for gush