Add *.gem to .gitignore to ignore gem build output
Update to Rakefile to use 'rdoc/task' instead of obsolete 'rake/rdoctask'
Add sequel, dm-sqlite-adapter and sqlite3 dependencies to gemspec and new Gemfile
Use the Sequel 4.0.0 after_initialize plugin to fix up failing Sequel tests

Due to the test model's use of "after_initialize", which was remove in Sequel
4.0.0, the credentials attribute of the model was not being initialized. This
change enables the after_initialize plugin to allow these tests to run.