Potential fix for #1057

Tests for the undo flag would definitely be a good idea, too.
don't undo any migrations if there aren't any
make sure that the executable does not break while undoing no migrations
typo
correctly pass the meta objects values to the options
formatting
test that migrations are correctly undone
Merge pull request #1229 from sequelize/hotfix/undo_migrations

Tests/Fixes for undoing migrations.
Merge branch 'issue_1057' of https://github.com/elliotf/sequelize into elliotf-issue_1057

Conflicts:
	bin/sequelize
run undo tests for the flags "--undo" and "-u" as well
sequelize --undo
better check if 'IS NOT' or '!=' should be used
added tests for postgres and sqlite
set the right context for postgres tests
Merge pull request #1231 from unlooped/master

better check if 'IS NOT' or '!=' should be used
Update changelog.md
Update README.md