mercurial: Install python bindings properly

Also allow to build with --HEAD by making brew's
download strategy aware of a brewed but unlined hg.

Adapted the caveats so that the procedure to brew
hg --HEAD is clear.

Mention how to config hgk in the caveats.

Library/Homebrew/download_strategy.rb

@@ -552,6 +552,7 @@ class MercurialDownloadStrategy < AbstractDownloadStrategy
     @path ||= %W[
       #{which("hg")}
       #{HOMEBREW_PREFIX}/bin/hg
+      #{Formula.factory('mercurial').opt_prefix}/bin/hg
       #{HOMEBREW_PREFIX}/share/python/hg
       ].find { |p| File.executable? p }
   end