Add a describe() method for resource meta information

[haydn]

var store = new Store();
store.define("products", {
  title: Store.attr(),
  category: Store.hasOne(),
  comments: Store.hasMany()
});
store.describe("products"); 
// {
//   fields: [ "title", "category", "comments" ],
//   attributes: [ "title" ],
//   relationships: [ "category", "comments" ],
//   hasOnes: [ "category" ],
//   hasManys: [ "comments" ],
//   pseudonyms: [ "products", "product" ]
// }