Create TaskCreds linkage

Migration for task_creds table
TaskCred model
Associations from Task and Cred through TaskCreds
Specs for Task, Cred and TaskCred
Task Factory

[Story #49167601]
TaskHosts association

Create TaskHost join table
Create associations
Migration
Specs

[Story #49167601]
TaskServices connector

Migration for TaskServices table
Model for Mdm::TaskService
Associations
Specs

[Story #49167601]
Add a few more specs to Mdm::Service

Mdm::Service had no specs, so added some additional coverage
Tests all associations and scopes on the model

[Story #49167601]
Add some YARD docs

Added YARD docs for associations

[Story #49167601]
incremement version #
Add dependent destroy

Make sure we kill association objects when either end is destroyed

[Story #49167601]
Fix namespacing issdue

[Story #49167601]
Fix to disallow duplication of Task* objects

Validate uniqueness on all Task join objects

[Story #49167601]
Merge pull request #27 from rapid7/feature/task_relations

Related Pro and MSF PRs passing, specs and functional passing on this.
Fix class name
Merge branch 'feature/task_relations'
Merge branch 'master' into feature/module-caching

Conflicts:
	lib/metasploit_data_models/version.rb
	spec/dummy/db/schema.rb