Updated Migration DSL

This is an updated version of #132.

TODO

- [x] Add more column types (boolean, decimal, etc..), currently only primary key, varchar, datetime, integer
- [x] Add `timestamps/0` function to DSL (maybe this should be a separate PR, since the models should support this notion too)
- [x] Shortcuts for `alter table(..)` (add_column, remove_column, modify_column, rename_column)
- [x] Existence functions (column_exists?, table_exists?, index_exists?)
- [ ] Support old string-based migrations by checking return value, not sure if this is needed because old migrations can be updated to use `execute/1`
- [ ] Column options, `default`, `scale`, `precision`, others?
- [x] Add `references/1` for creating foreign keys i.e.

```elxir
create table(:products) do
  references(:category)
end
```

It supports a DSL syntax, for creating/dropping/altering objects:

## Change Example:

```elixir
defmodule Repo.Migrations.CreatePosts do
  use Ecto.Migration

  def change do
    create table(:posts) do
      add :title, :string
      add :published_at, :time
    end
  end
end
```

## Up/Down Example:
```elixir
defmodule Repo.Migrations.CreatePosts do
  use Ecto.Migration

  def up do
    create table(:posts) do
      add :title, :string
      add :published_at, :datetime
      timestamps
    end
  end

  def down do
    drop table(:posts)
  end
end
```

## Alter Example:
```elixir
defmodule Repo.Migrations.AlterPosts do
  use Ecto.Migration

  def change do
    alter table(:posts) do
      add :summary, :string
      remove :published_at
      modify :title, :string
      rename :owner, :created_by
    end
  end
end

```
We need to explicitly document all commands since adapters are Ecto's public API. :)