Implement validate function

[roll]

Overview

There should be a function to validate datapackage descriptor against datapackage profile schema:

async validate(descriptor, profile='base', remote_profiles=False) -> raise/true

It should raise list of errors. See - https://github.com/frictionlessdata/jsontableschema-js#validate

This function should encapsulate work with different datapackage profiles (register) - we should decide how based on datapackage-py experience. This function will be used inside DataPackage class.

Tasks

• [x] finish https://github.com/frictionlessdata/datapackage-js/issues/14
• [x] see example - https://github.com/frictionlessdata/jsontableschema-js/blob/master/src/validate.js
• [x] consider using some validation code from - https://github.com/frictionlessdata/datapackage-validate-js
• [x] implement validate function (see API above)
• [ ] write comprehensive tests using data dir (real datapackage descriptors)