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 (ie, don't act before any merge).

Happy to keep working on this & tweaking it as necessary. If it's easier for the maintainers to close this and deal with it further down the line, I'll support that too. 
I think we can skip adding anything to `compat` here - they're here to provide compatibility for older formulae, which naturally wouldn't have been using these functions that didn't exist yet.