add some examples for the documentation on the nb_fetch utility

[mmcky]

this addresses issue #278.

When writing this, the default values set for this utility are probably no longer appropriate with the development of QuantEconLib. I propose to remove these defaults to make this utility a bit more general as a simple way of fetching files from a Github repo.

REPO = "https://github.com/QuantEcon/QuantEcon.notebooks"
RAW = "raw"
BRANCH = "master"
FOLDER = "dependencies"          #Hard Coded Dependencies Folder on QuantEcon.notebooks

[coveralls]

Coverage remained the same at 92.124% when pulling 483df52 on update-fetch-utility into a82ba69 on master.

[mmcky]

@cc7768 I am going to merge this PR. Let me know if you think the examples need to be updated and we can do so.