v3.0.0

Changelog:

Andrew Pillar (27):
      Add clean task to Makefile
      Rewrite for v3
      Add mysql init and open func
      Move database logic to top-level library
      Update readme with mgrt.Open usage
      Correctly set the node value for sorting
      Space out error message
      Expand tests
      Add comment
      gofmt -s -w
      Add link to Go doc
      Rewrite for v3
      Remove .thrall.yml
      Correctly link in version information
      Log revision comment and check for -type and -dsn flags
      Update readme
      Check for database type before trying to open the connection
      Use a struct with callbacks for abstracting database details
      Order by id as well as performed_at
      Add -n to mgrt log to limit the number of revisions to show
      Add mgrt show command for viewing individual revisions
      Reorder readme
      Fix test
      Add build manifest
      Rename migrations to revisions
      Update .gitignore
      Expand tests