Add cache location support to CVS.

This allows formulaes which use CVS to support:
  brew --cache [formula]

Library/Homebrew/download_strategy.rb

@@ -204,9 +204,15 @@ class GitDownloadStrategy <AbstractDownloadStrategy
 end
 
 class CVSDownloadStrategy <AbstractDownloadStrategy
+  def initialize url, name, version, specs
+    super
+    @co=HOMEBREW_CACHE+@unique_token
+  end
+
+  def cached_location; @co; end
+
   def fetch
     ohai "Checking out #{@url}"
-    @co=HOMEBREW_CACHE+@unique_token
 
     # URL of cvs cvs://:pserver:anoncvs@www.gccxml.org:/cvsroot/GCC_XML:gccxml
     # will become:
@@ -220,14 +226,13 @@ class CVSDownloadStrategy <AbstractDownloadStrategy
         safe_system '/usr/bin/cvs', '-d', url, 'checkout', '-d', @unique_token, mod
       end
     else
-      d = HOMEBREW_CACHE+@unique_token
-      puts "Updating #{d}"
-      Dir.chdir(d) { safe_system '/usr/bin/cvs', 'up' }
+      puts "Updating #{@co}"
+      Dir.chdir(@co) { safe_system '/usr/bin/cvs', 'up' }
     end
   end
 
   def stage
-    FileUtils.cp_r(Dir[HOMEBREW_CACHE+@unique_token+"*"], Dir.pwd)
+    FileUtils.cp_r Dir[@co+"*"], Dir.pwd
 
     require 'find'
     Find.find(Dir.pwd) do |path|