mirror support: External command for mirror tests

`brew mirror-check <formula...>` will process a list of Formulae and check
their mirrors to see if they are reachable and the MD5 sums are valid.

Library/Contributions/examples/brew-mirror-check.rb

+require 'formula'
+
+class Formula
+  def test_mirror mirror
+    url, specs = mirror.values_at :url, :specs
+    downloader = download_strategy.new url, name, version, specs
+
+    # Force the downloader to attempt the download by removing the tarball if
+    # it is allready cached.
+    tarball_path = downloader.tarball_path
+    tarball_path.unlink if tarball_path.exist?
+
+    begin
+      fetched = downloader.fetch
+    rescue DownloadError => e
+      opoo "Failed to fetch from URL: #{url}"
+      return
+    end
+
+    verify_download_integrity fetched if fetched.kind_of? Pathname
+  end
+end
+
+module Homebrew extend self
+  def check_mirrors
+    mirror_check_usage = <<-EOS
+Usage: brew mirror-check <formulae ...>
+
+Cycle through mirror lists for each formula, attempt a download and validate
+MD5 sums.
+    EOS
+
+    if ARGV.empty?
+      puts mirror_check_usage
+      exit 0
+    end
+
+    formulae = ARGV.formulae
+    raise FormulaUnspecifiedError if formulae.empty?
+
+    formulae.each do |f|
+      if f.mirrors.empty?
+        opoo "#{f.name} has no mirrors"
+        next
+      else
+        oh1 "Testing mirrors for #{f.name}"
+        f.mirrors.each{ |m| f.test_mirror m }
+      end
+    end
+  end
+end
+
+# Here is the actual code that gets run when `brew` loads this external
+# command.
+Homebrew.check_mirrors