Merge tag '1.6.1' into develop

1.6.1
Preparation for new development cycle after release 1.6.1
Fix sanity check for release track command.
Update changelog for issue #105
Update of the contributers.
Version bump 1.7.0-dev.5

Closes #105
Merge branch 'feature/issue_105' into develop
Refactor function gitflow_override_flag_boolean
Refactor function gitflow_override_flag_string
Update changelog
Version bump 1.7.0-dev.10
Merge branch 'feature/refactor-flag-override-functions' into develop
Rename function gf_determine_boolean.
Bugfix: newline unexpected (expecting ")")
Bugfix: newline unexpected (expecting ")")
Add shortcut command finish
Add shortcut command delete
Shortcut for finish and delete commands.

Introduce two new commands, git flow finish and git flow delete.
These commands will finish/delete the current branch you are on.
Version bump 1.7.0-dev.18

Closes #74
Merge branch 'feature/issue_74' into develop