First class support for the official registry

[pwalsh]

We have the first two options here, I'd like to introduce the 3rd, which is a simple flag to choose between the local cache and the official, live, remote registry over http.

# use the local cache of the official registry
registry = datapackage_registry.Registry()

# get any old registry
registry = datapackage_registry.Registry('http://someplace.com/my-registry.csv')

# work in front of the official registry instead of the local cache
registry = datapackage_registry.Registry(local=False)

[vitorbaptista]

Moved issue to https://github.com/datapackages/datapackage-py/issues/48