Use fetcher class implementation. Resolves #8

Depends on yahoo/fetchr#5
Will change the fetchr git-url to the published version once we merge above and publish 

@redonkulus @mridgway @lingyan 
I thought config was supposed to be an implementation detail for the client-side fetchr.

I am in favor of leaving context to minimize the breaking-changes/refactoring for fetchers.

We'll discuss offline in a bit.