Suppress the logging of the seed fetch by default

Add in handling of a task argument for the fetch task so that if the
logging is needed, you can run `rake db:seed DEBUG=true` or `rake
db:seeds:fetch DEBUG=true` in order add the logger to the Faraday
middleware stack.

With this change, here's the new output:
![screen shot 2014-07-30 at 11 17 30 pm](https://cloud.githubusercontent.com/assets/77174/3759931/018bba64-186a-11e4-9968-21424b7cf0bb.png)

And a snippet of the output when you run `rake db:seed DEBUG=true`:
![screen shot 2014-07-30 at 11 18 15 pm](https://cloud.githubusercontent.com/assets/77174/3759937/1a4717e2-186a-11e4-93f9-7dc31dd4c00a.png)


Thoughts? Happy to see this go either way. I think it's slightly dubious, but I did get to learn more about Rake, so it's not a total loss if it doesn't get merged in. :smiley: