Make valdator.js methods chainable.  Update add method interface.

* Fix bug with "isLength" validator default message.

* Update interface of methods from validator.js so that they produce
  chainable objects.  Add specs for this and update existing tests.

* Use local mocha binary.

* Add repository type to package.json.

* The "#add()" method interface now only takes a function, and stores
  that in an xtrudr instance's "general" property.