Add alias support to formulae

* brew install will find an aliased formula
* aliases are searched against
* warn when creating a new formula that has an existing alias.

If Subversion has an alias "svn", then warn when the user tries to
create a new formula "svn". The formula can still be created, though
the user should make sure it's not a duplicate of the existing
aliased one.

Subversion and Objective-Caml formulas get some alises here, so we have
something to test against.

Formula/objective-caml.rb

@@ -4,6 +4,8 @@ class ObjectiveCaml <Formula
   @url='http://caml.inria.fr/pub/distrib/ocaml-3.11/ocaml-3.11.1.tar.bz2'
   @homepage='http://caml.inria.fr/ocaml/index.en.html'
   @md5='fe011781f37f6b41fe08e0706969a89e'
+  
+  aka :ocaml, 'o-caml'
 
   def install
     system "./configure --prefix #{prefix}"

Formula/subversion.rb

@@ -17,6 +17,8 @@ class Subversion <Formula
   url 'http://subversion.tigris.org/downloads/subversion-1.6.6.tar.bz2'
   homepage 'http://subversion.tigris.org/'
   md5 'e5109da756d74c7d98f683f004a539af'
+  
+  aka :svn
 
   # Only need this on Snow Leopard; for Leopard the deps package 
   # builds it.