scripts to maintain parallel OS X branch

Synopsis:

- `scripts/create-osx.sh` -- creates a branch with the same name as the current branch with suffix `-osx`, and changes the `language` definition in `.travis.yml` in that branch
- `scripts/bump-osx.sh` -- merges the latest changes from the current branch into the `-osx` branch

Both scripts require a clean working directory and reset to the current branch after completion.