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)
[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

Migrate installer to Homebrew organisation:
https://github.com/mxcl/homebrew/pull/24355

Migrate web page to Homebrew organisation:
https://github.com/Homebrew/homebrew.github.io/pull/10