allow IssueTakeCommand to choose git remote

Org is also used for configuring the adapter
so unless a remote with the same name as org exists you can't
use this command
Merge pull request #357 from sstok/hotfix-issue-take

hotfix for IssueTakeCommand
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
add switch-base and squash pull request when merging