Create homebrew formula for jenv

The previous attempt to create Homebrew formula was rejected https://github.com/Homebrew/homebrew/pull/17406

But we have shown that jenv is a solid tool with many improvements since the last pull request. Let's follow Homebrew project's suggestion of creating our own homebrew formula - hence my pull request of the formula. 

Once this formula is merged, I will attempt to submit a new pull request to Homebrew project. Alternatively we can submit a pull request to https://github.com/Homebrew/homebrew-headonly