fix enable-sasl-pwdb

* using --enable-sasl-pwdb will make the compile fail with:

sasl_defs.c:148:4: error: initialization from incompatible pointer type [-Werror]
This adds the "shutdown" command to the server. This allows for better
automation (at least,... I find that it makes for better automation).
Fix make maintainer-clean
add aclocal 1.12 in autogen.sh
Added libevent's event_base_loopbreak in sig_handler method to allow memcached to properly shutdown
Merge branch 'master' of https://github.com/emcconville/memcached