deprecate setup script and add a composer.json

The setup script was kind of janky, so let's get rid of it and replace it with composer.json.

In this pull request, we switch to relying on the 1.5.0 `hhvm/xhp` library (@ptarjan has ownership over this as well as me and I'm hoping he will find it an appropriate home). We also note that this is an hhvm project.

This pull request also (sadly) pulls in the `hhi` directory. I feel like this is an okay compromise because it should go into the package install ultimately and not into individual applications since it is a direct product of the compilation process and so tied to the exact HHVM version. When that happens we can just nuke the `hhi` directory entirely.

Last, but not least, if and when `<?php // decl` lands, a new version of the XHP libraries that uses that notation instead should be shipped.

Let me know if you hate any of it! I've also added a deploying to Heroku readme after the main one. I'll send pull requests as we streamline that process.