Usergrid Services Layer

This is where most of the high-level functionality for accessing Usergrid data
is performed. It functions as the glue between the REST API as exposed by
Jersey and the entity management functionality in usergrid-core. This is also
where security and account management functionality is contained.

The security model of Usergrid is implemented with Apache Shiro:

http://shiro.apache.org/

At the moment, the permissions checks haven't been wired in, so once a user
is authenticated, they have full authorizations.  This will be fixed in
subsequent releases.

The services model for Usergrid involves converting the REST web service
requests into collection queries and routing these to a set of collection
handlers.  This provides a "virtual collection" layer on top of the actual
collections that are exposed in the persistence tier.  This makes it possible
to implement logic behavior on top of persistent objects, to enforce object
and property visibility, and to perform fine-grained permission checks.