Save user password as a bcrypt hash

* Replace password field by password_hash
* Add User#password attribute
* Implement password hashing and verification with BCrypt mixin

db/migrate/20110809130610_add_password_hash_to_users.rb

@@ -0,0 +1,11 @@
+class AddPasswordHashToUsers < ActiveRecord::Migration
+  def self.up
+    add_column :users, :password_hash, :string
+    remove_column :users, :password
+  end
+
+  def self.down
+    remove_column :users, :password_hash
+    add_column :users, :password, :string
+  end
+end