Support datapackage-identifier spec and load data packages from all those identifiers

[rufuspollock]

At the moment require explicit path to datapackage.json.

We should support all identifiers in http://dataprotocols.org/data-package-identifier/ and support loading this way e.g.

dp = DataPackage('.')
dp = DataPackage('./my-data-package')
dp = DataPackage('https://github.com/datasts/gdp')

We also need to store this info internally so we can compute path offsets for resources etc - see frictionlessdata/datapackage#75

[pwalsh]

👍

[roll]

Based on the latest direction of the development of the spec - frictionlessdata/frictionlessdata.io#875 - we're closing it for now, as Data Package Identifier might be deprecated (URL as the only standard location pointer instead of Datahub/Github/etc dependencies)