Feature: Define RoleManager as interface

[kenjones-cisco]

Define RoleManager as interface, and rename existing RoleManager struct
to defaultRoleManager to provide a default implementation and backwards
compatability.

Defines RoleManagerConstructor to allow specifying a function for
creating a new RoleManager.

Adds SetRoleManagerConstructor method to Model to allow setting the
function to call for a new instance of RoleManager.
Adds implementation of RoleManagerConstructor to Model that will create
an instance of the defaultRoleManager just like before.

Updates Assertion to reference RoleManager interface instead of an
instance. Updates Assertions.buildRoleLinks() to use newRoleManagerFunc
that holds the RoleManagerConstructor.

All test are passing with the only change being *RoleManager to
RoleManager.

Resolves #36

[coveralls]

Coverage decreased (-0.2%) to 91.543% when pulling 8514f98 on kenjones-cisco:feature/role-manager-interface into 748b815 on casbin:master.

[hsluoyz]

I have several questions:

1. I saw newRoleManagerFunc in model.go is a global variable. So two Casbin enforcers will affect each other. I think it should be a member of Enforcer to avoid the conflicts.

2. defaultRoleManager can be refactored out to default_role_manager.go. role_manager.go will only contain the interface.

3. The function comments in RoleManager interface didn't describe the domain parameter.

[kenjones-cisco]

If the RoleManagerConstructor is added to the Enforcer, that means that func (model Model) BuildRoleLinks() and func (ast *Assertion) buildRoleLinks() would need to accept a RoleManagerConstructor as input given that func (ast *Assertion) buildRoleLinks() creates a new RoleManager instance on each execution.

Any concerns with change the signatures of those methods?

[coveralls]

Coverage decreased (-0.2%) to 91.551% when pulling cd99ceb on kenjones-cisco:feature/role-manager-interface into 748b815 on casbin:master.

[hsluoyz]

The code looks good to me. Can you add a working example (e.g. TestRBACModelWithCustomRoleManager()) to model_test.go? In it, you can start with the RBAC example, then set a custom role manager. The role manager can be very simple, such as using hard-coded values. Then call testEnforce to show how the new role manager affects the enforcing results.

func TestRBACModel(t *testing.T) {
	e := NewEnforcer("examples/rbac_model.conf", "examples/rbac_policy.csv")

	testEnforce(t, e, "alice", "data1", "read", true)
	testEnforce(t, e, "alice", "data1", "write", false)
	testEnforce(t, e, "alice", "data2", "read", true)
	testEnforce(t, e, "alice", "data2", "write", true)
	testEnforce(t, e, "bob", "data1", "read", false)
	testEnforce(t, e, "bob", "data1", "write", false)
	testEnforce(t, e, "bob", "data2", "read", false)
	testEnforce(t, e, "bob", "data2", "write", true)
}

[coveralls]

Coverage increased (+0.06%) to 91.812% when pulling ef81f10 on kenjones-cisco:feature/role-manager-interface into 748b815 on casbin:master.