Configurable Validator Models

[nonplus]

Now that we can specify property models (#72), it would be great to be able to configure restifyValidation to automatically apply a model based on which validator is configured. That is, if a property uses { isInt: true } validation, its value would be automatically converted to a number without having to explicitly specify { model: Number }.

This PR adds a validatorModels configuration option and provides standard validatorModels for the built-in validators (isBoolean, isDate, isDecimal, isDivisibleBy, isFloat, isInt, isNatural). The README has more info.

The following example uses restifyValidation.validatorModels but then overrides the model for the isDate validator to map values to moment rather than to Date:

var moment = require('moment');
server.use(restifyValidation.validationPlugin({
    validatorModels: [
        restifyValidation.validatorModels,
        { isDate: moment }
    ]
}));

[nonplus]

@gchauvet, this is the last of my changes. With this, rest handlers will be able to work with typed data without having to modify the validation configuration for the individual routes.

Once this has been merged, would you mind doing a build and publishing to npm? Thanks!

[gchauvet]

@nonplus okay I merge changes in master branch. I will it release in 1 week max under version 1.2.0
Tank you for your contribs :)