Manage users via REST API and config files

[rjrudin]

As a ML developer
I want to TITLE
So that I don't have to create users manually or define a task for each one

• Example - https://github.com/marklogic/marklogic-samplestack/tree/master/database/security/users
• One user per file
• (src dir)/security/users
• Create a UsersManager that exposes public methods for each of the user endpoints at http://docs.marklogic.com/REST/management/security
• Once we can create a user, we should adjust our test framework to use a user with the manage-admin role, and possibly the rest-admin role for the LoadModulesCommand

[rjrudin]

Scott - can you continue this work on a branch named issue-3?