Tidy up formula and add postgresql option

- Update README to reflect change in repository ownership.
- Remove redundant version definition (homebrew scans the version automatically from the URL).
- Switch URL to tar.gz instead of zip, as per warning from `brew audit`.
- Clarify swig dependency as build only.
- Use preferred head url definition style (see homebrew pull request [20276](https://github.com/Homebrew/homebrew/pull/20276)).
- Use `build.with?` instead of `ARGV.include?` to check options.
- Add option to build postgresql cartridge.