Initial migrations support
Merge pull request #77 from 0xAX/migrations

Initial migrations support
Preload should work with assoc
Fix comments and rename migrations module
Merge pull request #79 from grjohan/fix-migrations

Fix docs and rename migratable module
Move migratable to ecto/adapter directory
Update README.md
Fully support both string and binary types

Elixir binary literals will be of string type by default and can be
casted to binary with the binary/1 function in Ecto.Query.API.
Work around for integer_to_binary/2 bug
Update migrator.ex
Update migrator.ex
Fix AssociationNotLoadedError error message
Fix preloader bug for belongs_to associations

Preloader did not consider that one associated entity may match multiple
parent entities for belongs_to.
Add more tests for assoc selector on belongs_to