More options to make delegate more flexible

Adds two options to delegate to make it more flexible and support two frequent patterns in delegation.

You can specify a target if you don't want your method to have the same name:

class Foo < ActiveRecord::Base
  belongs_to :greeter
  delegate :hi, :to => :greeter, :target => :hello
end

Foo.new.hi # returns Foo.new.greeter.hello

If the object in which you delegate can be nil, you may want to use the :allow_nil option. In that case, it returns nil instead of raising a NoMethodError exception:

class Foo
  def initialize(bar = nil)
    @bar = bar
  end
  delegate :zoo, :to => :bar
end

Foo.new.zoo   # raises NoMethodError exception (you called nil.zoo)

class Foo
  def initialize(bar = nil)
    @bar = bar
  end
  delegate :zoo, :to => :bar, :allow_nil => true
end

Foo.new.zoo   # returns nil