Migrate to Homebrew organisation

Rewrites all references to mxcl/homebrew and rewrites the remote on
`brew update`.

You can see a test repository named `testbrew` on the organisation:
https://github.com/Homebrew/testbrew

This has been migrated from my personal GitHub account.
The following Git clones were tested and follow the redirects:
git://github.com/mikemcquaid/testbrew.git
git@github.com:mikemcquaid/testbrew.git
https://github.com/mikemcquaid/testbrew.git

The following links were tested and follow the redirects:
https://github.com/mikemcquaid/testbrew
https://github.com/mikemcquaid/testbrew
https://github.com/mikemcquaid/testbrew/issues/1
https://github.com/mikemcquaid/testbrew/wiki/WHY-TESTBREW-SUX

This description will shortly be edited with links to other pull requests for the installer and web page.