Support datapackage identifier spec

[roll]

Overview

Specification - http://dataprotocols.org/data-package-identifier/
Code to merge - https://github.com/frictionlessdata/datapackage-identifier-js

[pwalsh]

@roll do we support this in the python lib?

[roll]

@pwalsh
Not yet - frictionlessdata/datapackage-py#76

So created it also here for a future.