Add Cloud Foundry Deploy scripts

Hi,

We added [Cloud Foundry](http://cloudfoundry.org/index.html) (CF) deploy scripts. The process involves two steps:

- **Staging**: We use a [Heroku Buildpack](https://devcenter.heroku.com/articles/buildpacks) to build a slug with the R environment and all its dependencies. Since the container does not have all the native dependencies, we had to pre-build them using the `scripts/cf-tar-dependencies.sh` bash script. The Heroku Buildpack runs the `init.r` script to build the specific RCloud dependencies.
- **Running**: The `manifest.yml` declares how to run the app on the separate running container. It must run in non-daemon mode or CF will consider the process dead.

We added a `doc/CF_INSTALL.md` with further details on how to execute this deploy.

Some caveats:
- This is an initial pass and does not include Solr, Redis, Python and GitGists.
- We tried using the `scripts/build.sh` script but this does not seem to download the CRAN dependencies. Maybe we were doing something wrong?
- To run `devtools::install` on `rcloud.shiny` it needed a `NAMESPACE` file. 
- To run `devtools::install` on `githubgist` it needed to declare `github` in its `DESCRIPTION` file.

Please provide feedback and let us know what you think.

@tlabeeuw & @gerg