Added pyenv formula

Added formula to install pyenv, a tool for installing and managing
multiple python versions. Based on rbenv and ruby-build.

Library/Formula/pyenv.rb

@@ -0,0 +1,29 @@
+require 'formula'
+
+class Pyenv < Formula
+  homepage 'https://github.com/yyuu/pyenv'
+  url 'https://github.com/yyuu/pyenv/tarball/v0.1.2'
+  sha1 '6703b7044d64814bed6003074c0f8be8c11504d1'
+
+  head 'https://github.com/yyuu/pyenv.git'
+
+  def install
+    prefix.install Dir['*']
+
+    var_lib = "#{HOMEBREW_PREFIX}/var/lib/pyenv"
+    ['plugins','versions'].each do |dir|
+      var_dir = "#{var_lib}/#{dir}"
+      mkdir_p var_dir
+      ln_sf var_dir, "#{prefix}/#{dir}"
+    end
+  end
+
+  def caveats; <<-EOS.undent
+    To enabled shims and autocompletion add to your profile:
+      if which pyenv > /dev/null; then eval "$(pyenv init -)"; fi
+
+    To use Homebrew's directories rather than ~/.pyenv add to your profile:
+      export PYENV_ROOT=#{opt_prefix}
+    EOS
+  end
+end