Adding define functions

[jwoertink]

This is a precursor to #393

This PR adds the ability to define postgres functions. By default I have made these trigger functions because I feel those may be the most common. A trigger function is a function that can only be called from a trigger. A trigger is just basically a callback in postgres.

Now, postgres functions can get super complicated real quick. There's a ton of possible options you can have, and all kinds of return types. What this PR does is gives you the ability to do some simple functions by providing you with some boilerplate. Beyond the simple ones you may need you can always use the execute() method in your migrations to hand roll your own complex functions.

The ability to define triggers will come in another PR.

def migrate
  # simple trigger function
  create_function "set_updated_at", <<-SQL
    IF NEW.updated_at IS NULL OR NEW.updated_at = OLD.updated_at THEN
      NEW.updated_at := now();
    END IF;
    RETURN NEW;
  SQL
end

def migrate
  # more complex example
  create_function "increment(i integer)", <<-SQL, returns: "integer"
    RETURN i + 1;
  SQL

  execute("SELECT increment(1)") #=> returns 2
end

[paulcsmith]

Looks great!