Allow building authorized_keys file using existing public keys

* allow usage of extra_ssh_keys attribute in user data bag item
* update provider to manage extra_ssh_keys
* add users_test cookbook for testing purposes
* cover ssh_keys and ssh_extra_keys with unit tests
* update README

Conflicts:
	providers/manage.rb
Add user's ssh_public_key to authorized_keys file by default

* prevent from inserting duplicates into authorized_keys
* cover with tests
Move test_ssh_keys test to other tests' dir
Merge pull request #1 from 37s-cookbooks/add-extra-ssh-keys

Add extra ssh keys