Added pagination

[eelcoj]

Usage:

class Content::Post < Perron::Resource
  configure do |config|
    config.pagination.per_page = 10
  end
end

class Content::PostsController < ApplicationController
  include Perron::PaginateHelper

  def index
    @paginate, @resources = paginate(Content::Post, Content::Post.all)
  end
end

<%# app/views/content/posts/index.html.erb %>
<%= link_to "Previous", @paginate.previous if @paginate.previous? %>
<%= link_to "Next", @paginate.next if @paginate.next? %>

Not 100% happy with the API yet—but pretty close for basic pagination. This design should also make it easy to extend with “pagination groupings”, eg. by year/month/day.

Closes #63