prosody: Add prosody v0.8.2

Sources:
http://wiki.openwrt.org/doc/howto/xmpp.server
https://dev.openwrt.org/ticket/12178
luaexpat: Add luaexpat v1.2.0

Note:
This is part of the prosody-v0.8.2-patch.

Sources:
http://wiki.openwrt.org/doc/howto/xmpp.server