Bookmarker: Stage 2
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Congratulations!  You now have a functioning web app
that integrates with the database and allows you to 
create, edit, and delete basic bookmarks.  But that 
was easy.  Now comes the part where you have to
write some Ruby!

Obviously, a list of URLs is not very sexy, so we're
going to do a few things.  First, we're going to add
a title, description, and date of creation field to
our bookmarks.  Next, we're going to add validations
to the bookmark model to enforce basic rules like
making sure the url is syntactically valid and that
required fields aren't empty.  Finally, we're going
to use ERB to tweak the bookmark views so the list
of bookmarks is a bit less generic looking.

So let's get started.  In order to add the fields to
the database in a manner we can track and fix if we
goof up, we need to create a migration.  This can be
performed by a ruby script from the command line:

ruby script/generate migration add_bookmark_fields

That will create a file "002_add_bookmark_fields.rb"
in your db/migrate/ directory.  Navigate there and
open up the file.  Once again you'll see a self.up
and a self.down method just like the first 
migration.  Now, you just need to figure out how to
add three columns: title, description, and 
created_at.  The title is a field type of :string, 
the description is type :text, and created_at is
type :timestamp.  In fact, created_at is a special
field name that Rails understands will be auto-
populated with the creation date and time.

Remember to write code in self.down that rolls back
the changes you make to the database in self.up!  
Once you finish, try your migration script out by
running at the command prompt:

rake db:migrate

If you're successful, the fields are added!  They
should even show up automatically in the views - 
refresh the listing in the browser, for instance.
Now open app/models/bookmark.rb to continue.

*** END OF STAGE 2 INTRO ***