Async support

[garygreen]

With this PR it adds asynchronous validation support:

Validator.registerAsync('username', function(value, ruleValue, attribute, passes) {
  // check from database etc then call...
  passes();
  // or fail with custom error
  passes(false, 'Username is banned');
}, ':attribute field is invalid');

It also refactors rules, .validate is no longer accessible, you need to access rules by Validator.getRule(<name>) which exposes some functions like .validate()

[garygreen]

@skaterdav85 I've got it all working now, lots of tests and all passing. The implementation could be tweaked a bit but if your happy with it / any suggestions let me know :-)

Btw I decided to go with the one callback passes as having two is a bit redundant and unnecessary. You could name this however you like in your app e.g. response(true) response(false, 'Custom error').

In any case we can work a few things out or tweak them before releasing 2.0.

[garygreen]

@skaterdav85 Have you had chance to take a look? :)

[iamdtang]

awesome work @garygreen!