Make sure urls start with http://

Made it so that the database does not accept a url unless it
begins with http.

Did it using activerecord validations, so this is a ruby-side validation,
not a database constraint.
tried out different ways of validating urls

Ended up using regexp from before, but tried out different ways from
railsguide and stack overflow. Still running into a problem where http://
and https:// act as valid urls. Not checking to make sure that the rest of the
url is valid.
Adjusted confirmation view to display meaningful error message

View now lets users know when they have entered an invalid url in the
same page that the shortened url was presented. That way the user knows that
something is wrong and can return to the home page.