Improve release process with Rake

Adds a series of rake tasks to improve the overall experience when
creating a new release

Note that because `Rake::PackageTask` generates its list of files to
package at the time of task generation (that is, as soon as you run
`rake`), you  _can not_ vendorize the gem dependencies inside the same
run as packaging the release. For that reason, I'm including a
`release.sh` shell script that vendorizes the gems, and then (in a
separate run of rake) packages the release.

`release` is the main task. It will run all of the tasks in the
`release` namespace in order

`release` namespace:
* `build` builds the new release and adds it to the destination
  repositories. (`tarball:build` and `homebrew:build`)
* `push` pushes the sub-repositories to GitHub (`tarball:push` and
  `homebrew:push`)
* `clean` removes all of the build artifacts from the repo
  (`gems:clean`, `tarball:clean`, and `homebrew:clean`)

`gems` namespace:
* `vendorize` will run the vendorize script, pointing at vendor/gems
* `clean` removes vendor/gems if it exists

`homebrew` namespace:
* `build` runs `checkout`, `formula:build`, and `commit`
* `checkout` checks out thoughtbot/homebrew-formulae as a local
  subdirectory
* `commit` adds a new commit to thoughtbot/homebrew-formulae for the new
  release
* `push` pushes thoughtbot/homebrew-formulae to master
* `clean` removes the homebrew formula directory if it exists

`homebrew:formula` namespace:
* `build` generates a formula for the current version from the template.
  It will replace `__VERSION__` with the current version, and `__SHA__`
  with the `shasum` of the packaged tarball. This formula will be
  written directly into the gh-pages subdirectory.

`tarball` namespace:
* `build` runs `checkout`, `package`, `move`, and `commit`
* `checkout` checks out the `gh-pages` branch of liftoff into a local
  subdirectory
* `move` moves the tarball generated by `package` from `pkg/` into the
  `gh-pages` subdirectory
* `commit` adds a new commit to the `gh-pages` branch of liftoff for the
  new release
* `push` pushes the `gh-pages` branch of liftoff up to GitHub
* `clean` removes the `gh-pages` and `pkg` subdirectories if they exist
* `package` is generated by `Rake::PackageTask`. It creates a new
  directory inside `pkg` for the current release, and creates a tarball
  out of it.
Add contributing file

Includes instructions on packaging a new release version