Fix for elixir v0.10.3

Ecto should try to be compatible with latest released Elixir when
possible.
Use Postgrex as the postgresql adapter
Update README and examples to use Postgrex
Dont overload + and - with dates

Closes #81.
Add loaded?/1 to associations

Closes #82.
Fix preloader for empty lists

Closes #90.
Update to Elixir master
Initial support migrations via mix tasks #80
fixes for #85
tests fix
Update postgrex dep
Improve migration handling
Fix the build, improve types
Improve migrator tests