Merge pull request #23 from davetoxa/4.0

Парочка исправлений
Merge pull request #24 from davetoxa/4.0

Update anatomy-of-a-migration
update readme
Update README.md
Merge pull request #26 from davetoxa/patch-2

Update README.md
Update source/01-rails-database-migrations/4-changing-existing-migrations.md
Merge pull request #27 from davetoxa/patch-3

Update source/01-rails-database-migrations/4-changing-existing-migration...
Merge remote-tracking branch 'upstream/4.0' into 4.0