List the options that the user has for installing Mercurial

Fixes Homebrew/homebrew#136

Library/Homebrew/download_strategy.rb

@@ -207,6 +207,12 @@ end
 
 class MercurialDownloadStrategy <AbstractDownloadStrategy
   def fetch
+    raise "You must install mercurial, there are two options:\n\n"+
+          "    brew install pip && pip install mercurial\n"+
+          "    easy_install mercurial\n\n"+
+          "Homebrew recommends pip over the OS X provided easy_install." \
+          unless system "/usr/bin/which hg"
+
     ohai "Cloning #{@url}"
     @clone=HOMEBREW_CACHE+@unique_token