Update default name lookup url & make configurable

[larrysalibra]

From #5 (comment)

The name look up URL you're using is a temporary one used by the explorer that will be removed. Since we're trying to encourage people and services to run their own Blockstack nodes, I made the blockstack-ruby library default to the node running on localhost and configurable to another host. In blockstack.js, we have the user pass in the name lookup URL instead of providing a default, It would be cool if we can do something like that in the python library as well. If you think it makes more sense to hardcode a default publicly accessible name look up url, https://core.blockstack.org/v2/users/{name} is a better option.