Shorthand for nested objects

[erfanium]

With this shorthand, validating a nested object will be easier.
old:

schema = {
   a: {
      type: "object",
      props: {
         b: {
            type: "object",
            props: {
               c: "string"
            }
         }
      }
   }
}

new:

schema = {
   a: {
      $$type: "object",
      b: {
         $$type: "object",
         c: "string"
      }
   }
}

[coveralls]

Coverage remained the same at 100.0% when pulling 5a4da85 on erfanium:master into 5ad4fb9 on icebob:master.

[icebob]

Interesting feature, thanks!