escalus:story - flexible resource counts

Now it's possible to selectively start a client in escalus:story,
e.g. start only the second one of all clients defined in test.config.

Instead of:
    (1) escalus:story(Config, [2,1], F).
Use:
    (2) escalus:story(Config, [{mary,3}], F).
With the same config (1) would start all defined clients, the first with
two resources, while (2) only the client named 'mary' with three
resources.
Initial presence is a story hook with default
Merge pull request #2 from lavrin/elastic_story_users

escalus:story - flexible resource counts
Allow escalus_stanza functions to use short jids

All escalus_stanza functions that took client as
parameter now can take atom with user name to
use short jid of that user

pair programmed with Michał Nieć
Fixed escalus_pred asserts for privacy lists
Added escalus_client:{username,server,resource}/1
Added escalus_config:get_config_with_fallback/2,3
Bugfix in escalus_new_assert:mix_match/2
Removed hooks, introduced overridables
Added iq_get/2 and iq_set/2 stanzas
Make post_story_checks return ok on no checks
Created escalus:assert_many/2
Added escalus:peek_stanzas/1
Improved escalus_assert:has_no_stanzas/1
Cleaner escalus_client:start/3
escalus_ejabberd:with_local_option/3 cleans better
Bugfix in escalus_story:make_everyone_friends/2
Added escalus_users:get_server/1
Allow modifying resources
Fix in escalus_ejabberd:get_c2s_status/1