Sequel specs

It looks like DatabaseCleaner's code paths for Sequel [aren't tested as much][pendingspec] as the more popular ActiveRecord code paths.

I'm relatively familiar with Sequel and various kinds of automated testing. I'm new to the DatabaseCleaner codebase, though, so I'm trying to keep in line with the existing tests. I'm also avoiding making any changes to the code under test (for now).

I'd appreciate feedback, particularly on the following topics:

 * [x] integration testing (does it work when we run it for real?) vs implementation testing (do these methods get called?)
 * [ ] modules and namespaces
 * [x] ruby 1.8 compatibility
 * [x] allowing one-shot `Truncation.new(db: foo)` instead of `t = Truncation.new; t.db = foo`

This pull request only covers truncation, but it is ready to merge. Although adding specs found some misbehaviours in the existing code, I'll leave fixing those problems for a separate pull request.


[pendingspec]: https://github.com/bmabey/database_cleaner/blob/v1.1.1/spec/database_cleaner/sequel/base_spec.rb#L27