Generate homebrew formula (wip)

* Doesn't fill in the SHA information yet.
Build tarball

* Vendorizes gem dependencies
* Checks out gh-pages as a subdirectory
* Builds tarball
* Moves tarball into place
Wrap release tasks in a namespace
Remove vendorizing task from build task

It looks like the package task builds the list of files when the task is
defined, so any files created as a part of the build process can't be
included in the tarball.
Add SHA to github formula
Use the liftoff executable from src