Add documentation for sometimes rule, and fix nested sometimes bug

[paulhbarker]

Prior to this change, if the sometimes rule was present on a nested property like so...

let rules = {
  'user.firstname': 'sometimes|required|string',
  ...
}

...the check to determine whether that nested property had data and needed to be validated was using Object.prototype.hasOwnProperty(), but "attributes" are delivered to the _suppliedWithData() method as "paths" in dot notation. This change fixes that by adding logic to determine if the "path" exists in the object, rather than passing a dot-notation string to Object.prototype.hasOwnProperty(), thus enabling the validation of nested properties with the sometimes rule.

A short sometimes rule addition was made to README.md as well 😄