Add/update/delete permission targets.

:construction: 

WIP for supporting permission targets.

- [ ] tests

cc @opscode/release-engineers 
Either way is fine with me.  When I started working on an HWP, I thought that this might be easier to read and work with:

````ruby
perm = Artifactory::Resource::PermissionTarget.new
perm.name = 'MyPerm'
perm.include_pattern = nil
perm.excludes_pattern = ''
perm.repositories = ['ANY']
perm.users = {
  'anonymous' => ['r']
}
perm.groups = {
  'readers' => ['r']
}
perm.save
````
than

````ruby
perm = Artifactory::Resource::PermissionTarget.new
perm.name = 'MyPerm'
perm.include_pattern = nil
perm.excludes_pattern = ''
perm.repositories = ['ANY']
perm.principals = {
  { 
    'users' => {
      'anonymous' => ['r'],
    }
    'groups' : {
      'readers' => ['r']  
    }
  }
}
perm.save
````