General pattern for manager classes

A manager class is one that's used to talk to a noun-based section of the Management API as documented at http://docs.marklogic.com/REST/management. Each section of the API should have a manager class, and some may have multiple manager classes because the noun represented by the section encompasses several lower-level nouns. For example, the Databases and Forests sections should each have a manager class, while the Security section would have manager classes for users, roles, amps, and the other nouns encompassed by "security".