Don't reset entity when manually loading association
Add `Validator.bin_dict/2`
Document reflection functions
Document association reflections
Add explicit transaction rollback with values

Thanks to @mururu for first implementation.
Fix Repo.delete/1 docs

Closes #169.
Update migration docs

Closes #154.
Update to Elixir v0.12.3
Association join's on expr should be able to use all bindings

Closes #171.
Add only one entry into schema_migrations per migration
Update to elixir master and use R16B02 on travis
Add `Repo.log/2`

Enables logging and debugging of adapter actions.
Merge pull request #174 from elixir-lang/repo-adapter-hook

Add `Repo.log/2`
Do not check type on `Entity.association/2` updates

Previous check was for a model record, which is wrong, the check should
be for the entity record. Since we cannot get the entity type from the
model at compile time the check should be removed altogether.
Update ex_doc
Update Elixir requirement and remove outdated sigil syntax