Server should support more REST-like paths

[aravindet]

This is a nice-to-have for 1.0.

As a prerequisite, we should add a soft convention for naming indexes, e.g. as '$<index_name>`, then we can do:

• /posts?by=time&first=10&fields=slug,title,at,authors(first:1,name,avatar) should become:

  {
    'posts$time': [ { first: 10 }, {
      slug: 1, title: 1, at: 1,
      authors: [ { first: 1}, {
        name: 1, avatar: 1
      } ]
    } ]
  }

• GET /posts/123?fields=slug,title,at,author(name,avatar) should become:

  {
    'posts': { 123: {
      slug: 1, title: 1, at: 1,
      author: { name: 1, avatar: 1 }
    } }
  }

[aravindet]

This change should be in common/encode/url.js.