add get_groupinfo api

1. add get_groupinfo api
2. add groupmaster to push message
remove master attribute when GroupOwner is empty
should not treat error response as group message
add overlay vars for local development

run "rebar generate overlay_vars=local_vars.config"
to generate release for development environment.

we adds -smp to vm.args this time
keep <nick/> for presence subscription stanzas

when the contact is offline, the presence subscription stanza will be
stored and resend when the contact is online.
but it doesn't store the original packet; only part of the stanza will
be stored and others will be lost.
this commit doesn't address this issue entirely; it just stores one more
element - the <nick/> element

and it passes the original packet to the handler of the
"roster_in_subscription" hook which makes it possible for the handler to
store the original packet

one more thing, we will only supports odbc, so mod_roster and
mod_shared_roster_ldap are not maintained
filter user sent messages

1. remove the <sender/> element if it's included
2. add timestamp to a message
filter out duplicated jids in a group
Merge branch 'groupchat_bugfix'
Merge branch 'dev'