Allow configuration of SleekXMPP's whitespace_keepalive_interval

Some servers/networking setups may require more agressive keep-alives to be
sent. This exposes SleekXMPP's configuration of this behaviour by exposing
it as an optional configuration item (XMPP_KEEPALIVE_INTERVAL).

This also makes err-agressive-keepalive obsolete again, so it has been
removed from repos.py.

closes #154
Merge pull request #155 from zoni/issue/154

Allow configuration of SleekXMPP's whitespace_keepalive_interval