Add HOMEBREW_NO_GITHUB_API env var.

This patch allows users to "opt out" of using the GitHub API altogether
by setting the HOMEBREW_NO_GITHUB_API environment variable. The
value of the environment variable does not matter (it can even be
empty!).

For Bash/ZSH: export HOMEBREW_NO_GITHUB_API=1

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
create: detect name from GitHub archives.
rbenv-readline 1.0.0 (new formula)

Fixes https://github.com/tpope/rbenv-readline/issues/1.
doctor: require CLT on 10.9.

Xcode-only setup is badly broken and will remain that way until fixed by
Apple. For many 10.9 issues this fixes things so let's tell the users.