Add support for pattern matching property names (#204)

This is an implementation of the second API example I proposed on #204. You use it like this:

```javascript
var schema = Joi.object()
schema.matchKeys(/_age$/, Joi.number().min(0))

// will return error for person_age key
Joi.validate({dog_age: 3, person_age: -12}, schema)
```

I'm not married to the exact API or method names, but this does meet my needs and serves as a starting point for discussion if nothing else.