Add links for requests to base URL and /servers.
Add /replica_sets links.
Add /sharded_clusters links.
Add apps/links.py
Restrict the scope of some of the links. Add at least basic links in sub-documents.
Store all links as global constants in links.py.
Git didn't catch on that links.py should go in the package after rebasing.
Merge pull request #105 from lovett89/rest-links-3

Add links to sub-resources and related actions in JSON responses.