Initial work for deb/rpm packages

Please note that this is a work in progress that hasn't been tested yet.

- [x] directory structure
- [x] initd support
- [x] post install & pre remove scripts
- [x] ~~package the rack gem~~ depend on the ruby rack package
- [x] RPM dependencies (at least for Fedora)
- [ ] test the built DEB on Debian / Ubuntu: [prax_0.9.0_amd64.deb](https://dl.dropboxusercontent.com/u/53345358/prax_0.9.0_amd64.deb)
- [ ] test the built RPM on Fedora / CentOS