do not start nor stop cowboy in mod_metrcis

it is done in the lister section
fixes #212
Merge pull request #231 from esl/mod_metrics-cowboy-fix

Do not start nor stop cowboy in mod_metrcis
introduced option allow_multiple_sessions
wip detecting new session of an existing occupant
wip change default value of allow_multiple_sessions to false, check if it is set
wip add dictionary of occupants sessions in a room
wip fix indentation
minor refactoring - introduce presence status codes
use existing function for status codes
wip change routing to given nick
fix; compare ljids instead of jids
fix commas
fix - send presence <<unavailable>> to all only in last session
fixes for version without multiple sessions allowed
fix update sessions dict on kicking out