title: Create migration in gem
body: <p>I decided to create my first gem in ruby and I have a problem. If a user adds my gem to their gemfile, I would like to create a new table in the database.</p>
  
  <p>I believe the only solution is to create a rake task which runs a custom script with the migration. Am I right? Do you know of any tutorials on modifying a database using plain Ruby? How do I create this script? Is there any other solution?</p>