Initial Yosemite Support

Very initial, tentative Yosemite support for Homebrew. This will almost
certainly remain something of a work in progress until Yosemite is
actually released, but for now should start the process of supporting
10.10. As ever, I’ll add a note that I absolutely do not recommend
anyone goes ahead and sticks these changes on their system until the
maintainers feel comfortable with that idea.